Fix CI skip conditions in tests.yml

[blink1073]

References

Description

Tightens skip conditions in the CI workflow so only steps that actually require GitHub secrets are conditionally skipped.

Changes

• Removed overly broad if condition from build job (was skipping on scheduled runs outside the main repo)
• Removed if: github.event_name == 'pull_request' from test_release job so it runs on all triggers
• Removed allowed-skips: test_release from tests_check since test_release no longer skips
• Added if condition to the Codecov upload step to skip on fork PRs (where CODECOV_TOKEN secret is unavailable)
• Added missing persist-credentials: false to cover job's checkout step

Backwards-incompatible changes

None

Testing

N/A

AI usage

• Some or all of the content of this PR was generated by AI.
• The human author has carefully reviewed this PR and run this code.
• AI tools and models used: Claude Sonnet 4.6 (Claude Code)