Fixup head reference for CI keyboard count verification

[tzarc]

Description

Fixes #21246.

github.ref points at refs/pull/<pr_number>/merge -- see https://docs.github.com/en/actions/learn-github-actions/contexts#github-context

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[sigprof]

The suggested modification at least does not fail completely, although the check is not ideal (the PR may remove 1 keyboard and add 2 new keyboards at the same time, and the keyboard count check would succeed).

BTW, some other things probably can be checked here automatically:

• If any keyboards are removed, the PR must target develop. (This would catch even cases when the old keyboard name becomes an alias, because qmk list-keyboards resolves both DEFAULT_FOLDER and keyboard_aliases.hjson aliases, so any old names that became aliases would be detected as removed even if rules.mk with DEFAULT_FOLDER remains at the old location.)
• If any keyboards are removed, the old names should remain valid as aliases (in this case the removed names might not appear in the output of qmk list-keyboards --no-resolve-defaults if the aliases are only in keyboard_aliases.hjson; not sure what would be the best check for this, but it should be possible).