verify_submission.py: delay git clone

[totalspectrum]

Cloning the VK-GL-CTS repository can take a long time, so defer doing that until at least some of the sanity checks have been done (e.g. that we know the tar file is legal). This prevents the user from having to wait 5 minutes (or longer!) to discover that they've mistyped the file name.

[CLAassistant]

All committers have signed the CLA.

[dgkoch]

If you have a local copy of the repo checked out, you can also use the -s argument to avoid cloning it completely.

I'm not necessarily objecting to the proposed change, but if you have a local repository, the new flow is worse if you got the tar name correct, but the source folder incorrect.