ci: Cache Zephyr workspace installation.

Can save several minutes downloading the Zephyr docker image and/or cloning
repo from GitHub. Cache keyed on the Zephyr version, which AFAIK is the
only determinant for the workspace contents.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

.github/workflows/ports_zephyr.yml

@@ -30,9 +30,22 @@ jobs:
         docker-images: false
         swap-storage: false
     - uses: actions/checkout@v4
+    - id: versions
+      name: Read Zephyr version
+      run: source tools/ci.sh && echo "ZEPHYR=$ZEPHYR_VERSION" | tee "$GITHUB_OUTPUT"
+    - name: Cached Zephyr Workspace
+      id: cache_workspace
+      uses: actions/cache@v4
+      with:
+        # note that the Zephyr CI docker image is 15GB. At time of writing
+        # GitHub caches are limited to 10GB total for a project. So we only
+        # cache the "workspace"
+        path: ./zephyrproject
+        key: zephyr-workspace-${{ steps.versions.outputs.ZEPHYR }}
     - name: Install packages
       run: source tools/ci.sh && ci_zephyr_setup
     - name: Install Zephyr
+      if: steps.cache_workspace.outputs.cache-hit != 'true'
       run: source tools/ci.sh && ci_zephyr_install
     - name: Build
       run: source tools/ci.sh && ci_zephyr_build