Run cirrus linux-docs only on presubmit. (#70141)

.cirrus.yml

@@ -152,15 +152,14 @@ task:
         - dart --enable-asserts ./dev/bots/test.dart
 
     - name: docs-linux # linux-only
-      # TODO(fujino): Make this pre-submit only once we have migrated docs generation to LUCI (https://github.com/flutter/flutter/issues/64212).
       environment:
-        # TODO(tvolkert): optimize CPU and MEMORY settings once docs generation is more efficient (https://github.com/flutter/flutter/issues/60646).
         CPU: 4
         MEMORY: 8G
         # For uploading master docs to Firebase master branch staging site
         FIREBASE_MASTER_TOKEN: ENCRYPTED[eb768d18798fdc5abfe09b224e1724c4d82831d715ccf90df2c79d618c317216cbd99493278361f6fe7948b409b603f0]
         # For uploading stable docs to Firebase public live site
         FIREBASE_PUBLIC_TOKEN: ENCRYPTED[37e8b82f167864cae9a3f4d2cf3f37dea331d9375c295327c45de524f6c588fa6f6d63e5784f10f6d43ce29689f36c92]
+      only_if: "$CIRRUS_PR != ''"
       script:
         - ./dev/bots/docs.sh