Use the directory name, not full directory in SwiftBuildSupport.SWIFT_REPO_NAME

[hughbe]

@gottesmm, from #8439

If this isn't correct, then I'll update the comments to fix #them.

The comments say:

Set SWIFT_REPO_NAME in your environment to control the name of the swift
directory name that is used.

However:

print(SWIFT_REPO_NAME)

C:\Users\hughb\Documents\GitHub\swift\swift

This is the full path, not the repo/directory name, which is not what this variable means.

[hughbe]

@swift-ci please clean smoke test

[gottesmm]

@hughbe SGTM!