The regression tests don't work on PRs from forks

[paddyroddy]

@connoraird looks like the regression tests don't work on PRs from forks.

Originally posted by @paddyroddy in #1137 (comment)

[connoraird]

Ah yes, that makes sense because we have the repo url hardcoded (I think).

[paddyroddy]

Yeah, I guess I'd never thought about it. I'll make a fix tomorrow and make a fork from @UCL-MIRSG or something as a test.

[connoraird]

I've been thinking about this and the naive approach feels like we allow the before/after_revisions to be passed in as full URLs. However, that made me think that this might be a security vulnerability. What do you think?

[paddyroddy]

I don't think we should pass URLs. I was wondering about using the GitHub CLI and doing, e.g. gh pr checkout.

The counterpoint is that we can't accept any PRs from forks in case they decrease performance.