Re-test and label/exclude modules with failing tests

Problem

When a widely-used module (e.g., untar, samtools/index) is modified, it triggers tests for all dependent modules and subworkflows. This frequently surfaces pre-existing test failures in modules that haven't been tested in a while, blocking PRs that are unrelated to those failures.

This creates a frustrating situation where contributors end up debugging and fixing other people's broken tests just to get their own changes merged, or requiring maintainers to override CI failures.

Recent example: https://github.com/nf-core/modules/pull/9796 - a topic channel migration for 5 modules triggered failures in galah, kraken subworkflows, and others that were already broken.

Proposed Solution

As agreed in the nf-core maintainers meeting, implement a periodic CI job (weekly/monthly?) that:

1. Runs all module/subworkflow tests regardless of changes
2. Labels modules with failing tests (e.g., tests-failing, needs-fix)
3. Optionally excludes known-failing modules from PR CI runs until fixed

This would:

• Surface broken tests proactively rather than when someone touches a dependency
• Make it clear which modules need attention
• Prevent unrelated failures from blocking PRs
• Provide a "health dashboard" for the modules repository

Additional Considerations

• Labels could trigger notifications to module maintainers
• A scheduled issue could be auto-created listing all failing modules
• Consider a "quarantine" mechanism where known-failing tests are skipped in PR CI but tracked separately