Don't strip '.bst' from user-supplied checkout dir

Resolves #1361 (closed)

There are two lines of code which strip ".bst" from the end of the name of the checkout directory.

This MR moves those two lines inside an 'if' block, which ensures they'll only apply when the user hasn't supplied a checkout directory path, and buildstream is choosing/creating a default location.

I believe this is the only scenario where those two lines are expected to apply.