ci: make git-diff-develop work for PRs from foreign repos

The gh API requires authentication, even if the API endpoints do not. We
can just fetch the PR URL directly without shelling out.

.circleci/scripts/git-diff-develop.ts

@@ -18,11 +18,8 @@ async function getBaseRef(): Promise<string | null> {
     return null;
   }
 
-  // We're referencing the CIRCLE_PULL_REQUEST environment variable within the script rather than
-  // passing it in because this makes it easier to use Bash parameter expansion to extract the
-  // PR number from the URL.
-  const result = await exec(`gh pr view --json baseRefName "\${CIRCLE_PULL_REQUEST##*/}" --jq '.baseRefName'`);
-  const baseRef = result.stdout.trim();
+  const pull = await (await fetch(process.env.CIRCLE_PULL_REQUEST)).json();
+  const baseRef = pull.base.ref;
   return baseRef;
 }