Re-add fork behaviour

We shouldn't have this in nb-tester since it's specific to our repo. It's short enough to inline in our action.

.github/workflows/notebook-test.yml

@@ -68,9 +68,24 @@ jobs:
           args = ["tox", "--", "--write"]
           if changed_notebooks and not changed_config:
               args.extend(changed_notebooks.split("\n"))
-          if os.environ["PR_REPOSITORY"] != os.environ["GITHUB_REPOSITORY"]:
-              args.append("--is-fork")
-          subprocess.run(args, check=True)
+          try:
+            subprocess.run(args, check=True)
+          except Exception as err:
+            if os.environ["PR_REPOSITORY"] != os.environ["GITHUB_REPOSITORY"]:
+                raise SystemExit(
+                  "We can't run this test on pull requests from forks."
+                  "\n\n"
+                  "If you have write access to Qiskit/documentation, push to a new "
+                  "branch there and make your pull request from that branch instead."
+                  "\n\n"
+                  "If you don't have write access, you must test out the notebook "
+                  "locally using the instructions in "
+                  "https://github.com/Qiskit/documentation#execute-notebooks."
+                  "When this PR is approved, a maintainer will merge it to a new "
+                  "branch in Qiskit/documentation, then make a PR from that branch "
+                  "into main so it can pass CI."
+                ) from err
+            raise err
 
       - name: Detect changed notebooks
         id: changed-notebooks