Graceful fallback when a git executable is not present

[mikepurvis]

Describe the bug

In CI setups, it's common to be in a very a minimal environment that's missing a git executable, but also working in a git clone that was volumed-in from a sidecar container. You can end up with Nix seeing the .git directory and then throwing this:

error: getting the HEAD of the Git tree '/home/jenkins/agent/workspace/<job>' failed with exit code 1:
       error: executing 'git': No such file or directory

However, if .git is deleted, then Nix happily treats it as a plain directory flake. It would be great if this were the automatic behaviour, so:

• See .git, try to invoke git executable.
• If missing, emit a warning and fall back to treating the directory as if there was no .git present.