ci: add Linux cache to improve build performance

Author: coenttb

.github/workflows/ci.yml

@@ -65,6 +65,12 @@ jobs:
     container: swift:6.2
     steps:
       - uses: actions/checkout@v5
+      - name: Cache Swift packages
+        uses: actions/cache@v4
+        with:
+          path: .build
+          key: ${{ runner.os }}-spm-${{ hashFiles('Package.resolved') }}
+          restore-keys: ${{ runner.os }}-spm-
       - name: Build
         run: swift build
       - name: Run tests