Do not checkout tests repo in nightly tests action

[jtraglia]

For some reason, the nightly test vectors generator action is packaging files like this:

version https://git-lfs.github.com/spec/v1
oid sha256:a2cf780b8dc8fe675a7b2f5a133dcf1331e7739032fb10dfab20a68a652a15e6
size 19

When reviewing the workflow, I noticed that we checkout this repo (which takes forever!) unnecessarily. And I think that's the cause of the problem, because it's configured with Git LFS support.