Support default paths on Windows for custom clone path

What does this MR do?

Uses platform separators for default build and cache directories.

Why was this MR needed?

Custom build directories on Windows with shell executors fails with:

ERROR: Job failed: the GIT_CLONE_PATH="C:\\GLR\\builds\\gitlab-kitware-cmake ci" has to be within "C:\\GLR/builds"

because the default path has /, but Go's path routines are a bit silly and assume platforms only have a single directory separator.

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

This is a complete guess as to the solution to the problem. There are probably other places in the codebase which are assuming / works everywhere.

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 #4858 (closed)