build: bump pinned nixpkgs channel from nixos-25.11 to nixos-26.05

[jbw976]

Description of your changes

This PR makes the same bump to nixos-26.05 that crossplane/crossplane#7715 does and then follows the same conventions to make updates and handle the fall out from tool bumps that come along with the nixos bump, e.g. updated linters.

The details in crossplane/crossplane#7715 apply here too 🤓

I have:

• Read and followed Crossplane's contribution process.
• Run ./nix.sh flake check to ensure this PR is ready for review.
• Added or updated unit tests.
• Linked a PR or a docs tracking issue to document this change.
• Added backport release-x.y labels to auto-backport this PR.

Need help with this checklist? See the cheat sheet.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: dd3bb897-ad95-40fd-a49c-57c7190dbdaf

📥 Commits

Reviewing files that changed from the base of the PR and between c4f6622 and 973456b.

⛔ Files ignored due to path filters (8)

• apis/changelogs/proto/v1alpha1/changelog.pb.go is excluded by !**/*.pb.go, !**/*.pb.go and included by **/*.go
• apis/changelogs/proto/v1alpha1/changelog_grpc.pb.go is excluded by !**/*.pb.go, !**/*.pb.go and included by **/*.go
• apis/pipelineinspector/proto/v1alpha1/pipeline_inspector.pb.go is excluded by !**/*.pb.go, !**/*.pb.go and included by **/*.go
• apis/pipelineinspector/proto/v1alpha1/pipeline_inspector_grpc.pb.go is excluded by !**/*.pb.go, !**/*.pb.go and included by **/*.go
• apis/proto/v1alpha1/ess.pb.go is excluded by !**/*.pb.go, !**/*.pb.go and included by **/*.go
• apis/proto/v1alpha1/ess_grpc.pb.go is excluded by !**/*.pb.go, !**/*.pb.go and included by **/*.go
• flake.lock is excluded by !**/*.lock and included by none
• nix/checks.nix is excluded by none and included by none

📒 Files selected for processing (5)

• .golangci.yml
• flake.nix
• pkg/fieldpath/fieldpath.go
• pkg/fieldpath/merge.go
• pkg/reconciler/managed/metrics.go

🚧 Files skipped from review as they are similar to previous changes (5)

• pkg/fieldpath/merge.go
• flake.nix
• pkg/reconciler/managed/metrics.go
• pkg/fieldpath/fieldpath.go
• .golangci.yml

---

📝 Walkthrough

Walkthrough

The changes update lint and Nix tooling configuration, simplify field path formatting, modernize reflection checks, and centralize the managed-resource metric label constant.

Changes

Runtime consistency updates

Layer / File(s)	Summary
Lint and Nix tooling configuration .golangci.yml, flake.nix	Updates lint exclusions, the nixpkgs channel, and the development shell formatter.
Field path and reflection implementation updates pkg/fieldpath/fieldpath.go, pkg/fieldpath/merge.go	Writes field path segments directly to the builder and uses reflect.Pointer for pointer checks.
Managed-resource metric label consistency pkg/reconciler/managed/metrics.go	Centralizes the gvk label name for histogram definitions and label generation.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• crossplane/crossplane-runtime#961: Both changes update flake.nix Nixpkgs or toolchain configuration.

Suggested reviewers: negz

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is 66 characters, clearly describes the nixpkgs channel update, and stays below the 72-character limit.
Description check	✅ Passed	The description explains the nixpkgs bump and related tooling updates, which directly match the pull request changes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Breaking Changes	✅ Passed	No exported declarations changed. Non-generated Go changes preserve behavior; changed .pb.go files are generated and excluded by this check.

---

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[coderabbitai]

Note

GitHub couldn't provide a complete incremental comparison for this pull request, so CodeRabbit is performing a full review instead. This review may take a little longer.

[adamwg]

lgtm

[github-actions]

Backport failed for release-2.2, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-2.2
git worktree add -d .worktree/backport-1108-to-release-2.2 origin/release-2.2
cd .worktree/backport-1108-to-release-2.2
git switch --create backport-1108-to-release-2.2
git cherry-pick -x 973456b6c2cb63460ca70e9796ac6093669cb9c5

[github-actions]

Backport failed for release-2.3, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-2.3
git worktree add -d .worktree/backport-1108-to-release-2.3 origin/release-2.3
cd .worktree/backport-1108-to-release-2.3
git switch --create backport-1108-to-release-2.3
git cherry-pick -x 973456b6c2cb63460ca70e9796ac6093669cb9c5