test_coverage workflow fails on external PRs

[paulb777]

See #7472

The test_coverage workflow should either be disabled or made to work for PRs from forked repos.

[granluo]

If secrets are not able to be accessed, then Metrics Service cannot be used and so code coverage report is not able to be created. To avoid failing test workflows, we might need to disable this workflows under certain situations.
I made some experiments and it looks like the sdk repo can run workflows from a forked repo, not sure if secrets can be used or not yet. Will make more experiments.

[paulb777]

Reopening. Still seeing failures in #7507

[paulb777]

@granluo Still seeing failures in https://github.com/firebase/firebase-ios-sdk/pull/7549/checks?check_run_id=1921393904

Please test in a fork.

[granluo]

Have test on a fork repo. The corresponding step is in the step check files of the job Check changed files
https://github.com/firebase/firebase-ios-sdk/runs/1929701292?check_suite_focus=true

The failure is probably caused by that fork repo not syncing to the upstream yet.

[paulb777]

I see thanks.

[paulb777]

Reopening - the issue looks back in #7955

See https://github.com/firebase/firebase-ios-sdk/pull/7955/checks?check_run_id=2424517247

[paulb777]

@granluo This is still failing in #8483 and #8484. Please test on a forked PR. Reopening.

[paulb777]

There is still a post-submit issue, but that's not a big deal. I'll close since presubmits are now working.