Make tests callable from publishing-api.

Publishing API has [a pre-merge workflow][1] that calls these tests, so we
need to be explicit with the checkout repo/ref.

[1]: https://github.com/alphagov/publishing-api/blob/3fdb7c2/.github/workflows/test-content-schemas-1.yml#L108-L113

.github/workflows/rspec.yml

@@ -21,6 +21,8 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
         with:
+          repository: alphagov/collections
+          ref: ${{ inputs.ref || github.ref }}
           show-progress: false
 
       - name: Checkout Publishing API (for Content Schemas)