Limit Sigstore signer concurrency #469

After checking upstream (sigstore/sigstore-python#1403) with TUF and sigstore-python, it seems any concurrency limitation needs to be on our end.

Since Sigstore is the default, the TUF fetching code is invoked when the tests call signing.Config().

I happens very rarely on my development machine and more often in the github actions actually. I got this here:

E       FileExistsError: [Errno 17] File exists: 'root_history/12.root.json' -> '/home/stefanb/.local/share/sigstore-> python/tuf/https%3A%2F%2Ftuf-repo-cdn.sigstore.dev/root.json'

If feels like this error is related to concurrency and could even show up if someone ran multiple instances of the model_signing library while creating signatures concurrently. If so, the TUF library should probably have a lock/lock-file that prevents concurrency inside this function.

Originally posted by @stefanberger in #465 (comment)