craft/checkout-repository: chop off .git from repo name

If the repository URL ends in .git (https://invent.kde.org/group/foo.git),
checkout-repository.py assumed that the repository name (and thus the local
directory of the clone) would be "foo.git". Since the "foo.git" directory
didn't exist locally, it tried to clone the repository again, which failed
because "foo" did exist.

Fix by chopping off the final .git from repositoryName.