Fix gitlab-ce#4431 by removing uselessly prohibitive directory check.

What does this MR do?

This MR fixes a long standing bug in the gitlab runner that prevents caching in many useful cases.

Why was this MR needed?

Because people like to cache artifacts outside of the build directory and those artifacts are often not easy to move around.

Are there points in the code the reviewer needs to double check?

Sure, consider any security implications of allowing the code committer to push any file on disk to cache. Perhaps there should be a gitlab runner config option to regain the old behavior some day.

Does this MR meet the acceptance criteria?

I'd rather think about the converse. Did the prior behavior meet the acceptance criteria? No, undocumented, no tests known.

What are the relevant issue numbers?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/4431 https://gitlab.com/gitlab-org/gitlab-ce/issues/36656