use shlex to parse/ escape stings for POSIX shells

[weriomat]

Use the shlex crate to split/ parse the ssh_opts instead of just splitting by ' '.
If shlex fails to parse/ join, we fall back to the old way.

Fixes #130. Might fix #343.

[heitor-lassarote]

Thank you for your work on this. I've rebased your branch (and resolved conflicts) and added some tests.

One note: I don't think this fixes #343. That failure happens during the direct SSH activation call, where each ssh_opts entry is already passed as its own argument, not through a joined string. The two spots this PR touches (the --ssh-opts CLI flag and NIX_SSHOPTS for remote builds) aren't on that code path. I'll leave #343 open separately.

CI is currently red. I didn't delve into it myself but I had Claude check. So please don't take its feedback at face value, just something that perhaps could help unblock this PR:

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join_ssh_opts (used at both NIX_SSHOPTS call sites in push.rs) uses shlex::try_join, which quotes more aggressively than it needs to. It wraps something like StrictHostKeyChecking=no in single quotes even though there's no space to protect.

That's a problem because Nix's NIX_SSHOPTS parsing only became quote-aware in Nix 2.26. Before that, Nix split the string with tokenizeString, a plain whitespace split with no quote or escape handling at all (confirmed in Nix 2.24.12's own source, SSHMaster::addCommonSSHOpts). This repo's pinned nixpkgs ships Nix 2.24.12, so the quoted value comes through as one token, quotes included, and ssh chokes on it: Bad configuration option: 'stricthostkeychecking. That matches the failing jobs exactly.

It's not limited to ProxyCommand-style values either. Since shlex quotes almost anything containing =, it affects nearly every ssh option, which is why most of the VM tests fail, not just the one aimed at a specific edge case.

A quoting function that only wraps a value in quotes when it actually contains whitespace, instead of delegating to shlex's more conservative default, would fix the space-in-a-value case on Nix 2.26+ without changing anything for values that already worked, on any Nix version. Worth noting: even with that fix, a value with an embedded space still won't work on Nix below 2.26, since that Nix has no way to receive it correctly at all. That's a Nix-side limitation, not something fixable from here.

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I don't have the bandwidth to implement that fix as part of this review, so flagging it here.