ci: Call make html directly

Since `docs/prep` is a prerequisite of `docs`, and should be the *only* prerequisite, calling `make docs` could end up hiding a problem with files missing from the uploaded artifact. Instead, call `make` from the docs directory which should be closer to what will run on RTDs.

.github/workflows/prepare-docs.yml

@@ -67,7 +67,7 @@ jobs:
       - name: Test build docs
         shell: bash
         run: |
-          make docs -j${{ env.procs }} TARGETS= EXTRA_TARGETS=
+          make -C docs html -j${{ env.procs }} TARGETS= EXTRA_TARGETS=
 
       - name: Trigger RTDs build
         if: ${{ needs.check_docs_rebuild.outputs.docs_export == 'true' }}