Attempt to fix Javadoc snapshots:

- google/truth#1413
- #1900
- google/jimfs#401

Compare cl/460733976 from google/guava#6106. I notice that that earlier CL also added a `permissions` section for the `test` job, both for reading (for `actions/checkout`) and for writing (for `styfle/cancel-workflow-action`, which apparently [we probably shouldn't be using nowadays](https://github.com/styfle/cancel-workflow-action/blob/main/README.md#:~:text=You%20probably%20don%27t%20need%20to%20install%20this%20custom%20action) and which [might not even need the explicit permissions](https://github.com/styfle/cancel-workflow-action/blob/main/README.md#advanced-token-permissions)?). I'm not worrying about that for now, but that offers further evidence that I have no idea what I'm doing here.

RELNOTES=n/a
PiperOrigin-RevId: 802346694

Author: cpovirk

.github/workflows/ci.yml

@@ -72,6 +72,8 @@ jobs:
         run: ./util/publish-snapshot-on-commit.sh
 
   generate_docs:
+    permissions:
+      contents: write
     name: 'Generate latest docs'
     needs: test
     if: github.event_name == 'push' && github.repository == 'google/auto'