chore(ci): improve GHCR cleanup workflow

[pront]

Summary

Improves the weekly GHCR cleanup workflow with three changes:

• Add a new step to delete dated nightly tags (e.g. nightly-2025-01-15-alpine) older than 30 days, with an explicit guard to never touch semver/release tags
• Increase test-runner cleanup batch size from 50 to 100 per run to drain backlogs faster
• Add continue-on-error: true to the test-runner step (was missing, unlike the vector step)

Vector configuration

N/A

How did you test this PR?

Reviewed the jq filter logic manually. The dated nightly deletion step has two guards:

1. Skip any version where any tag matches ^[0-9]+.[0-9]+ (release tags)
2. Only delete versions with a tag matching ^nightly-YYYY-MM-DD-* that are older than 30 days

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

N/A