Check environment-file and pip-install-target inputs

These are marked as optional in the inputs block. There was an implicit
assumption that it's required to have one of the inputs but not both.
This can't be encoded in the inputs block so check the values as the
very first step.

.github/workflows/docs-ci.yaml

@@ -55,6 +55,17 @@ jobs:
       run:
         shell: bash -l {0}
     steps:
+      - name: Check inputs
+        if: (inputs.environment-file == '' && inputs.pip-install-target == '') ||
+            (inputs.environment-file != '' && inputs.pip-install-target != '')
+        run: |
+          cat <<~~
+          This workflow requires one (and only one) of the following inputs:
+          - environment-file
+          - pip-install-target
+          ~~
+          exit 1
+
       - uses: actions/checkout@v4
         with:
           repository: ${{ inputs.repo }}