Apply suggestion from @jameslamb

Author: jameslamb

.github/workflows/pr.yaml

@@ -44,7 +44,7 @@ jobs:
       contents: read
     uses: rapidsai/shared-workflows/.github/workflows/compute-matrix.yaml@ci/docs-build-matrix
     with:
-      build_type: ${{ inputs.build_type || 'branch' }}
+      build_type: pull-request
       matrix_name: docs-build
   docs-build:
     needs: [docs-build-matrix, checks]