flaky CI setup: required-contracts-ci

`required-contracts-ci` required check within our CI is flaky due to infrastructure issues.

```
┌─ semver-diff
│ curl: (22) The requested URL returned error: 403
│ Error: failed to resolve target branch for PR #20739
└

✗ 1 check(s) failed (15 passed, 1 failed)

This run took 4 minutes 16 seconds
exit status 1
error: Recipe `check-fast` failed on line 461 with exit code 1
```

I had to restart it >5 times for it to pass.

---

Also it appears many of these jobs are downloading dependencies on every run - something we could potentially cache.

Start with the required-contracts-ci job and the check-fast recipe at line 461, then trace the semver-diff step that failed while resolving PR #20739. Reproduce the failure and inspect where dependencies are downloaded; done means the required check no longer flakes and the relevant downloads are cached if supported.

Written by the indexing model from the issue text.