update CI to use latest cache action

Updates tailscale/corp#31905

Signed-off-by: Percy Wegmann <[email protected]>

Author: oxtoacart

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
           echo "module=$(go env GOMODCACHE)" | tee -a $GITHUB_OUTPUT
 
       - name: Set up cache
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: |
             ${{ steps.cache.outputs.build }}