Fix writing diagnostics on Windows

[henrymercer]

Fix an issue where the CodeQL Action could not write diagnostics on Windows by removing colons from the generated filename so it is valid on Windows.

Merge / deployment checklist

• [ ] Confirm this change is backwards compatible with existing workflows.
• [ ] Confirm the readme has been updated if necessary.
• [ ] Confirm the changelog has been updated if necessary.

[henrymercer]

Checked that the diagnostics are being written correctly in the PR checks. We may want to add an automated test too, but I'm not sure it's worth it.