fix(ci): Use correct check for external repositories

[teor2345]

Motivation

There's a mistake in PR #7956 where it is skipping important jobs on all PRs in CI, rather than just external PRs.

PR Author Checklist

Check before marking the PR as ready for review:

• Will the PR name make sense to users?
• Does the PR have a priority label?
• Have you added or updated tests?
• Is the documentation up to date?

If a checkbox isn't relevant to the PR, mark it as done.

Specifications

https://docs.github.com/en/actions/learn-github-actions/contexts#github-context

You can see the full github.event object for an external PR here:
https://github.com/ZcashFoundation/zebra/actions/runs/6997671169/job/19034900585?pr=7999#step:2:995

Solution

Use the fork field rather than trying to guess from the PR branch name

Testing

I manually checked this worked:

• real workflow runs: https://github.com/ZcashFoundation/zebra/actions/runs/6997949380/job/19035477953?pr=8000
• patch-external skipped: https://github.com/ZcashFoundation/zebra/actions/runs/6997949377/job/19035476464?pr=8000

Since the logic is inverted for an external PR, that should also work.

Review

This is urgent because some PRs and Mergify are skipping some CI jobs.

Reviewer Checklist

Check before approving the PR:

• Does the PR scope match the ticket?
• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
• Are all the PR blockers dealt with?
  PR blockers can be dealt with in new tickets or PRs.

And check the PR Author checklist is complete.

[teor2345]

The lint error is a known issue that shouldn't block merging this PR:

Error: no schema with key or ref "http://json-schema.org/draft-07/schema#"

https://github.com/ZcashFoundation/zebra/actions/runs/6997949358/job/19035511113?pr=8000#step:5:13

[teor2345]

Failed due to #7898

[arya2]

Looks good, thank you!

[arya2]

Not sure what happened here, the schema at that URL seems to be there.

Run marocchino/validate-dependabot@v2.1.0
Error: no schema with key or ref "http://json-schema.org/draft-07/schema#"

https://github.com/ZcashFoundation/zebra/actions/runs/7008453149/job/19064814803#step:5:13

[teor2345]

Not sure what happened here, the schema at that URL seems to be there.

Run marocchino/validate-dependabot@v2.1.0
Error: no schema with key or ref "http://json-schema.org/draft-07/schema#"

ZcashFoundation/zebra/actions/runs/7008453149/job/19064814803#step:5:13

It seems to be a rate-limit caused by how that action is implemented internally.