bump github action cache to v4 (#8199)

We want to [migrate off GH actions/cache v1 by Feb 1
2025](actions/toolkit#1890). That's soon!

To test that using `actions/cache@v4` works, I
1.  edited `pr.yaml` to add a *Mount Bazel Cache* step
2. saw a PR build run successfully on an empty cache
3. pushed a no-op commit and saw the next PR build fetch the cache and
succeed

If it really is that simple I'll do the same thing for the internal
repo.

.github/workflows/main.yaml

@@ -18,7 +18,7 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Mount Bazel cache
-        uses: actions/cache@v1
+        uses: actions/cache@v4
         with:
           path: "/home/runner/repo-cache/"
           key: repo-cache