Use relative path in cache key

[ntkme]

This is an alternative solution to the issue described in #904 and #905.

The problem statement is that when use aws codebuild as self-hosted runner, the $GITHUB_WORKSPACE would change on every single build, and currently the cache key contains process.cwd(), thus it will always invalidate the cache.

The solution in this PR is to use a relative cwd computed from $GITHUB_WORKSPACE instead of the absolute cwd, so that it works consistently on github runner and other runners.

Note: This change will break all caches on all repositories once when updating to this version due to updating the cache key, but after that cache should work again.

• Fixes Bundler cache not working due to ephemeral workdir #904
• Closes Allow customising project ID in cache key rather than it always including working directory #905