ci: skip the coverage comment on pull requests from forks

[kikoso]

The "Jacoco Report to PR" step in the instrumentation workflow posts the coverage comment with secrets.SYNCED_GITHUB_TOKEN_REPO. Pull requests from forks do not get repository secrets, so the token is empty and the step fails with 'token' is missing, which marks run-instrumentation-test as failed even when every test passed. That happened on #1029.

This skips the step for fork pull requests only. The emulator tests still run and the reports are still uploaded as artifacts; same-repo PRs, workflow_dispatch and repository_dispatch runs keep posting the comment as before.

[github-actions]

Coverage (unit tests)

Compared against cb74115 on main (#1031).

Module	Line %	Change	Branch %	Change
maps-compose	0.00%	no change	0.00%	no change
maps-compose-utils	2.04%	no change	0.49%	no change
maps-compose-widgets	0.00%	no change	0.00%	no change
TOTAL	0.41%	no change	0.09%	no change

_{Line and branch coverage from unit test reports. History is recorded on the coverage-history branch after each merge to main.}

[googlemaps-bot]

Coverage (instrumentation tests)

Overall Project	44.45%	🍏

There is no coverage information present for the Files changed

[googlemaps-bot]

Code Coverage

Overall Project	44.43%	🍏

There is no coverage information present for the Files changed