CI: Save bootstrap cache right after bootrap

This avoids caching a corrupted cache in cases where a job modifies the
environment (e.g. the Darwin job hiding zap-cli). It also makes the cache
available slightly sooner.

Also add plumbing for splitting caches over buildjet and github backends.

.github/actions/bootstrap/action.yaml

@@ -12,47 +12,67 @@ inputs:
 outputs:
   cache-hit:
     description: "Bootstrap environment was restored from cache"
-    value: ${{ fromJSON(steps.bootstrap-cache.outputs.outputs).cache-hit }} # retry returns all outputs in `outputs`
+    value: ${{ fromJSON(steps.restore.outputs.outputs).cache-hit }} # retry returns all outputs in `outputs`
 
 runs:
   using: "composite"
   steps:
-    - name: Calculate bootstrap cache key
-      id: bootstrap-cache-key
+    - name: Determine bootstrap cache configuration
+      id: prepare
       shell: bash
       run: |
         # Calculate bootstrap cache key
         # In addition to the various setup files, the work directory matters as well,
         # because the bootstrapped Pigweed environment contains absolute paths.
+        echo "Calculating bootstrap cache key for '$PWD'"
         FILES_HASH="${{ hashFiles('scripts/setup/*', 'third_party/pigweed/**') }}"
         FINAL_HASH="$(echo "$PWD:$FILES_HASH" | shasum -a 256 | cut -d' ' -f1)"
-        echo "Calculated bootstrap cache key for '$PWD': $FINAL_HASH"
-        echo "hash=$FINAL_HASH" >> "$GITHUB_OUTPUT"
+        echo key="${RUNNER_OS}-${RUNNER_ARCH}-${{ inputs.platform }}-${FINAL_HASH}" | tee -a "$GITHUB_OUTPUT"
 
-    - uses: Wandalen/[email protected]
-      name: Bootstrap from cache
-      id: bootstrap-cache
+        # Define cache paths in one place (composite actions don't support a top-level env)
+        echo paths=".environment build_overrides/pigweed_environment.gni" | tee -a "$GITHUB_OUTPUT"
+
+        # Split caches across backends
+        case "$RUNNER_OS" in
+          macOS) echo backend=actions;;
+          *)     echo backend=buildjet;;
+        esac | tee -a "$GITHUB_OUTPUT"
+
+    - name: Bootstrap from cache
+      id: restore
+      uses: Wandalen/[email protected]
       continue-on-error: true
       with:
-        action: buildjet/cache@v4
+        action: ${{ steps.prepare.outputs.backend }}/cache/restore@v4
         attempt_limit: 3
         attempt_delay: 2000
         with: |
-          key: ${{ runner.os }}-${{ runner.arch }}-${{ inputs.platform }}-${{ steps.bootstrap-cache-key.outputs.hash}}
-          path: |
-              .environment
-              build_overrides/pigweed_environment.gni
+          key: ${{ steps.prepare.outputs.key }}
+          path: ${{ steps.prepare.outputs.paths }}
 
     - name: Run bootstrap
-      if: fromJSON(steps.bootstrap-cache.outputs.outputs).cache-hit != 'true'
+      if: ${{ fromJSON(steps.restore.outputs.outputs).cache-hit != 'true' }}
       env:
-        PW_NO_CIPD_CACHE_DIR: Y
+        PW_NO_CIPD_CACHE_DIR: 1
+        PW_ENVSETUP_NO_BANNER: 1
       shell: bash
       run: source scripts/bootstrap.sh -p all,${{ inputs.platform }}
 
-    - name: Uploading bootstrap logs
+    - name: Save bootstrap cache
+      uses: Wandalen/[email protected]
+      if: ${{ fromJSON(steps.restore.outputs.outputs).cache-hit != 'true' }}
+      continue-on-error: true
+      with:
+        action: ${{ steps.prepare.outputs.backend }}/cache/save@v4
+        attempt_limit: 3
+        attempt_delay: 2000
+        with: |
+          key: ${{ steps.prepare.outputs.key }}
+          path: ${{ steps.prepare.outputs.paths }}
+
+    - name: Upload bootstrap logs
       uses: actions/upload-artifact@v4
-      if: always() && !env.ACT && fromJSON(steps.bootstrap-cache.outputs.outputs).cache-hit != 'true'
+      if: ${{ always() && !env.ACT && fromJSON(steps.restore.outputs.outputs).cache-hit != 'true' }}
       with:
         name: ${{ inputs.bootstrap-log-name }}
         path: |