Add all PR checks to a workflow collection, then run after unit-tests job

[mbg]

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[mbg]

https://github.com/github/codeql-action/actions/runs/17880542596 suggests we are just about exceeding the limit for what we can do with this:

[Invalid workflow file: .github/workflows/pr-checks.yml#L77](https://github.com/github/codeql-action/actions/runs/17880542596/workflow)
error parsing called workflow
".github/workflows/pr-checks.yml"
-> "./.github/workflows/__all.yml"
: too many workflows are referenced, total: 51, limit: 50