ci: add SDK (extracted) sources to separate SDK cache

We cannot add it to the sources cache because we share the cache among
all variants and as the caches are immutable, the first one to finish
will decide the cache for all subsequent use.

This will prevent the SDKs from being saved and will cause failures in
the next step as it fetches the common cache, most likely saved by a
Linux variant and finds no cache.

Saving it in the source cache isn't a great option either because it
will get frequently invalidated. Better to just give it its own cache.

.github/workflows/build-depends.yml

@@ -53,6 +53,16 @@ jobs:
           restore-keys: |
             depends-sources-
 
+      - name: Cache SDKs
+        uses: actions/cache@v4
+        if: inputs.build-target == 'mac'
+        with:
+          path: |
+            depends/SDKs
+          key: depends-sdks-${{ hashFiles('depends/hosts/darwin.mk') }}
+          restore-keys: |
+            depends-sdks-
+
       - name: Restore cached depends
         uses: actions/cache/restore@v4
         id: restore
@@ -66,7 +76,12 @@ jobs:
             depends-${{ hashFiles('contrib/containers/ci/Dockerfile') }}-${{ inputs.build-target }}-
 
       - name: Build depends
-        run: env ${{ steps.setup.outputs.DEP_OPTS }} HOST=${{ steps.setup.outputs.HOST }} make -j$(nproc) -C depends
+        run: |
+          export HOST="${{ steps.setup.outputs.HOST  }}"
+          if [ "${HOST}" = "x86_64-apple-darwin" ]; then
+            ./contrib/containers/guix/scripts/setup-sdk
+          fi
+          env ${{ steps.setup.outputs.DEP_OPTS }} make -j$(nproc) -C depends
 
       - name: Save depends cache
         uses: actions/cache/save@v4

.github/workflows/build-src.yml

@@ -44,6 +44,15 @@ jobs:
           echo "PR_BASE_SHA=${{ github.event.pull_request.base.sha || '' }}" >> $GITHUB_OUTPUT
         shell: bash
 
+      - name: Restore SDKs cache
+        uses: actions/cache/restore@v4
+        if: inputs.build-target == 'mac'
+        with:
+          path: |
+            depends/SDKs
+          key: depends-sdks-${{ hashFiles('depends/hosts/darwin.mk') }}
+          fail-on-cache-miss: true
+
       - name: Restore depends cache
         uses: actions/cache/restore@v4
         with: