github-actions: cache netmap checkout

To avoid build failures due to rate limiting, cache the netmap checkout.

Explicitly restore and save to avoid the checkout action cleaning the
checkout up before it can be stored in the cache.

.github/workflows/builds.yml

@@ -2374,6 +2374,13 @@ jobs:
     needs: [prepare-deps, prepare-cbindgen]
     runs-on: ubuntu-22.04
     steps:
+      - name: Restore Cache Netmap
+        uses: actions/cache/restore@v4
+        id: netmap-cache
+        with:
+          path: netmap/
+          key: netmap-git
+
       # Cache Rust stuff.
       - name: Cache cargo registry
         uses: actions/cache@13aacd865c20de90d75de3b17ebe84f7a17d57d2
@@ -2429,12 +2436,20 @@ jobs:
                 linux-headers-$(uname -r)
 
       - name: Checkout Netmap repository
+        if: steps.netmap-cache.outputs.cache-hit != 'true'
         uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938
         with:
           repository: luigirizzo/netmap
           # gets cloned to $GITHUB_WORKSPACE/netmap/
           path: netmap/
 
+      - name: Save Netmap Cache
+        if: steps.netmap-cache.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v4
+        with:
+          path: netmap/
+          key: netmap-git
+
       - name: Compile and install Netmap
         run: |
           cd $GITHUB_WORKSPACE/netmap/LINUX