Preserve bazel repository cache in github actions (#14495)

Highlights:

- Trimmed down to just the repository cache for faster upload / download
- Made the cache key only include files that affect external
dependencies (since that's what the repository cache caches) -
MODULE.bazel, codex-rs/Cargo.lock, codex-rs/Cargo.toml
- Split the caching action in to explicit restore / save steps (similar
to your rust CI) which allows us to skip uploads on cache hit, and not
fail the build if upload fails

This should get rid of 842 network fetches that are happening on every
Bazel CI run, while also reducing the Github flakiness @bolinfest
reported. Uploading should be faster (since we're not caching many small
files), and will only happen when MODULE.bazel or Cargo.lock /
Cargo.toml files change.

In my testing, it [took 3s to save the repository
cache](https://github.com/siggisim/codex/actions/runs/23014186143/job/66832859781).

.github/workflows/bazel.yml

@@ -78,30 +78,17 @@ jobs:
         shell: bash
         run: ./scripts/check-module-bazel-lock.sh
 
-      # TODO(mbolin): Bring this back once we have caching working. Currently,
-      # we never seem to get a cache hit but we still end up paying the cost of
-      # uploading at the end of the build, which takes over a minute!
-      #
-      # Cache build and external artifacts so that the next ci build is incremental.
-      # Because github action caches cannot be updated after a build, we need to
-      # store the contents of each build in a unique cache key, then fall back to loading
-      # it on the next ci run. We use hashFiles(...) in the key and restore-keys- with
-      # the prefix to load the most recent cache for the branch on a cache miss. You
-      # should customize the contents of hashFiles to capture any bazel input sources,
-      # although this doesn't need to be perfect. If none of the input sources change
-      # then a cache hit will load an existing cache and bazel won't have to do any work.
-      # In the case of a cache miss, you want the fallback cache to contain most of the
-      # previously built artifacts to minimize build time. The more precise you are with
-      # hashFiles sources the less work bazel will have to do.
-      # - name: Mount bazel caches
-      #   uses: actions/cache@v5
-      #   with:
-      #     path: |
-      #       ~/.cache/bazel-repo-cache
-      #       ~/.cache/bazel-repo-contents-cache
-      #     key: bazel-cache-${{ matrix.os }}-${{ hashFiles('**/BUILD.bazel', '**/*.bzl', 'MODULE.bazel') }}
-      #     restore-keys: |
-      #       bazel-cache-${{ matrix.os }}
+      # Restore bazel repository cache so we don't have to redownload all the external dependencies
+      # on every CI run.
+      - name: Restore bazel repository cache
+        id: cache_bazel_repository_restore
+        uses: actions/cache/restore@v5
+        with:
+          path: |
+            ~/.cache/bazel-repo-cache
+          key: bazel-cache-${{ matrix.os }}-${{ hashFiles('MODULE.bazel', 'codex-rs/Cargo.lock', 'codex-rs/Cargo.toml') }}
+          restore-keys: |
+            bazel-cache-${{ matrix.os }}
 
       - name: Configure Bazel startup args (Windows)
         if: runner.os == 'Windows'
@@ -229,3 +216,14 @@ jobs:
             print_failed_bazel_test_logs "$bazel_console_log"
             exit "$bazel_status"
           fi
+
+      # Save bazel repository cache explicitly; make non-fatal so cache uploading
+      # never fails the overall job. Only save when key wasn't hit.
+      - name: Save bazel repository cache
+        if: always() && !cancelled() && steps.cache_bazel_repository_restore.outputs.cache-hit != 'true'
+        continue-on-error: true
+        uses: actions/cache/save@v5
+        with:
+          path: |
+            ~/.cache/bazel-repo-cache
+          key: bazel-cache-${{ matrix.os }}-${{ hashFiles('MODULE.bazel', 'codex-rs/Cargo.lock', 'codex-rs/Cargo.toml') }}