perf: optimize CI caching for faster builds

- Split deps and _build into separate caches for better granularity
- Add Elixir/OTP version to cache keys for version-specific builds
- Include lib/**/*.ex hash in build cache for change detection
- Add apt package caching to skip redundant system dependency installs
- Use version-type: strict in setup-beam for consistent versions
- Improves cache hit rate and reduces build times significantly

.forgejo/workflows/build.yaml

@@ -40,20 +40,38 @@ jobs:
         with:
           otp-version: '28.3'
           elixir-version: '1.19.5'
+          version-type: strict
+
+      - name: Cache system dependencies
+        uses: actions/cache@v4
+        id: system-deps-cache
+        with:
+          path: /var/cache/apt
+          key: ${{ runner.os }}-apt-${{ hashFiles('.forgejo/workflows/build.yaml') }}
+          restore-keys: ${{ runner.os }}-apt-
 
       - name: Install system dependencies
         run: |
           sudo apt-get update
           sudo apt-get install -y libsnmp-dev snmp-mibs-downloader
 
-      - name: Restore dependencies cache
+      - name: Cache Mix dependencies
         uses: actions/cache@v4
         with:
-          path: |
-            deps
-            _build
-          key: ${{ runner.os }}-mix-${{ hashFiles('**/mix.lock') }}
-          restore-keys: ${{ runner.os }}-mix-
+          path: deps
+          key: ${{ runner.os }}-mix-deps-${{ hashFiles('**/mix.lock') }}-elixir-1.19.5-otp-28.3
+          restore-keys: |
+            ${{ runner.os }}-mix-deps-${{ hashFiles('**/mix.lock') }}-
+            ${{ runner.os }}-mix-deps-
+
+      - name: Cache compiled build
+        uses: actions/cache@v4
+        with:
+          path: _build
+          key: ${{ runner.os }}-mix-build-${{ hashFiles('**/mix.lock') }}-${{ hashFiles('lib/**/*.ex') }}-elixir-1.19.5-otp-28.3
+          restore-keys: |
+            ${{ runner.os }}-mix-build-${{ hashFiles('**/mix.lock') }}-
+            ${{ runner.os }}-mix-build-
 
       - name: Install dependencies
         run: mix deps.get