add cache for build

.github/workflows/android.yaml

@@ -9,11 +9,56 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    env:
+      ENABLE_NIX_STORE_CACHE: 'true'   # set to 'true' to test caching the whole /nix/store (risky)
+      USE_CACHIX: 'false'              # set to 'true' to enable Cachix step
 
     steps:
       - name: Checkout code
         uses: actions/checkout@v6
 
+      - name: Cache Nix local cache (~/.cache/nix)
+        uses: actions/cache@v4
+        with:
+          path: ${{ env.HOME }}/.cache/nix
+          key: ${{ runner.os }}-nix-local-${{ hashFiles('flake.lock','flake.nix') }}
+          restore-keys: |
+            ${{ runner.os }}-nix-local-
+
+      - name: (Optional) Cache entire /nix/store (experimental, very large)
+        if: env.ENABLE_NIX_STORE_CACHE == 'true'
+        uses: actions/cache@v4
+        with:
+          path: /nix/store/
+          key: ${{ runner.os }}-nix-store-${{ hashFiles('flake.lock','flake.nix') }}
+          restore-keys: |
+            ${{ runner.os }}-nix-store-
+
+      - name: (Optional) Setup Cachix (use binary cache)
+        if: env.USE_CACHIX == 'true'
+        uses: cachix/cachix-action@v14
+        with:
+          name: inventree
+
+      - name: Cache Flutter Pub
+        uses: actions/cache@v4
+        with:
+          path: ${{ env.HOME }}/.pub-cache
+          key: ${{ runner.os }}-pub-${{ hashFiles('pubspec.lock','pubspec.yaml') }}
+          restore-keys: |
+            ${{ runner.os }}-pub-
+
+      - name: Cache Gradle
+        uses: actions/cache@v4
+        with:
+          path: |
+            ${{ env.HOME }}/.gradle/caches
+            ${{ env.HOME }}/.gradle/wrapper
+            android/.gradle
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/build.gradle*','android/gradle.properties','android/gradle/wrapper/gradle-wrapper.properties') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+
       - name: Prepare Nix devshell
         uses: ./.github/actions/nix-devshell
         with: