create git caches atomically (backport #11581)

[mergify]

When working on speeding up the CI,
I triggered a race condition in the creation of the tarball cache. This code now instead will ensure that half-initialized repositories are no longer visible to any other nix process.

This is the error message that I got before:

error: opening Git repository '"/Users/runner/.cache/nix/tarball-cache"': could not find repository at '/Users/runner/.cache/nix/tarball-cache'

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This is an automatic backport of pull request #11581 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 12d5b2c has failed:

On branch mergify/bp/2.24-maintenance/pr-11581
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 12d5b2cfa.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libfetchers/git-utils.cc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[Mic92]

Merge conflict solved in #11601