Sanitize user-provided custom build directory before passing it forward

What does this MR do?

Run user-provided custom build directory through path.Clean before returning it to other functions. Prevents undefined behavior that could arise when using GIT_CLONE_PATH as reference to build other directory paths.

Why was this MR needed?

Currently, TmpProjDir is built using a [possibly] user defined variable. The current implementation implies different behaviors depending on how that variable is defined, such as adding a folder level when a trailing slash is present.

What's the best way to test this MR?

• Define GIT_CLONE_PATH with and without a trailing slash
• Resulting TmpProjDir should be the same in both cases.

What are the relevant issue numbers?

Closes #28946 (closed)