Implement support for floating output paths

[poly2it]

I took a shot at resolving the issues with ca-derivations (#164) since I use them quite frequently. This resolves the issues I was having, but you might want to test the changes further. The changes were implemented with help from Claude Opus 4.7. If this is an issue, I would like to make you aware of it.

This branch keeps the user-facing API unchanged. A Nix transformation, applied in both flake and non-flake eval modes, walks nodes.*.profiles.*. When path evaluates to a derivation, it is transformed such that path becomes outPath and a sibling drvPath is added. The binary picks up the new drvPath field on ProfileSettings and builds the .drv directly. The realised output is then resolved by calling nix path-info <drv>^out against the local or remote store. That resolved path is used throughout copy, activate, confirm, and revoke, so no stage falls back to the eval-time placeholder.

[poly2it]

I pushed two additional commits to resolve an issue which was only made evident after initial support was implemented.

In Nix 2.34, path-info on a placeholder drv looked up its hash-modulo-keyed build trace directly and found the output. I updated to the Git version of Nix 2.35 for stability reasons, and found that the placeholder no longer has its own build-trace entry. Only the resolved derivation does, and the lookup must walk through the resolution step. The resolution does however still work through nix build, which is now used instead.

See this document in the Nix repository for an explanation to the changes:

https://github.com/NixOS/nix/blob/41b28ad2ed0f4dcde548fa9714136f714a7fd789/doc/manual/rl-next/build-trace-rework.md

[Sereja313]

Thanks!