Fix path separator for CI_PROJECT_DIR in Windows in bash shells

What does this MR do?

This fix decides whether to turn / into \ only if the path contains a Windows drive letter (e.g. C:/gitlab-runner or C:\gitlab-runner) and happens on a Windows OS.

Why was this MR needed?

To finish a community contribution (!1546 (closed))

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

I changed from using a regexp to leveraging filepath.VolumeName. The differences are:

• filepath.VolumeName takes into consideration UNC paths
• filepath.VolumeName only causes platformAppropriatePath to do any transformation if running a Windows runner. I'm assuming that's the only scenario where we'll want CI_BUILDS_DIR and CI_PROJECT_DIR to have these path changes.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

• closes #4524 (closed)
• closes #2841 (closed)