Ensure that builds within the cache aren't considered Git repositories

[charliermarsh]

Summary

Some packages encode logic to embed the current commit SHA in the version tag, when built within a Git repo. This typically results in an invalid (non-compliant) version. Here's an example from pylzma: https://github.com/fancycode/pylzma/blob/ccb0e7cff3f6ecd5d38e73e9ca35502d7d670176/version.py#L45.

This PR adds a phony, empty .git to the cache root, to ensure that any git commands fail.

Closes #1768.

Test Plan

• Create a tag on the current commit, like v0.5.0.
• Build pylzma, using a cache within the repo:

rm -rf foo
cargo run venv
cargo run pip install "pylzma @ https://files.pythonhosted.org/packages/03/d8/10ef072c3cd4301a65a1b762b09eefa02baf8da23b9ea77ebe9546499975/pylzma-0.5.0.tar.gz" --verbose  --cache-dir bar

[zanieb]

Nice.

Relatedly, should we include an empty .gitignore in the checkouts bucket?

[charliermarsh]

Yeah, that seems wise.

[charliermarsh]

Perhaps worth mentioning that this does mean people can’t version the cache and check it into Git (I don’t think?).

[charliermarsh]

I guess the “best” version of this would be to put the .git in the temporary directory when building, but then omit it when we persist the build to the cache.