We ran into this issue wherein GitHub's `actions/checkout` would fail
because `git-config` would fail to include additional configuration.
The symptom was:
```
Error: fatal: could not read Username for 'https://github.com': terminal prompts disabled
```
And is caused by `git config --local includeIf.gitdir:/var/lib/github-runners/_work/…`.
Wherein `git` apparently resolves (canonicalizes) the current repository
path which then does not match the argument for `includeIf.gitdir` which
then means the configuration that `actions/checkout` is trying to apply
does not get pulled in, which then prevents git from authenticating with
GitHub and fails the build.
Ngl, gemini 3 found that out for me.
We could prefix `/private` everywhere, but changing the user's home
directory is going to [be tricky], not sure what do to about that.
[be tricky]: 7e22bf538a/modules/users/default.nix (L208)
a58dd30d2b