ci: use SwiftPM caches

Author: kkebo

.github/workflows/ci.yml

@@ -9,6 +9,14 @@ jobs:
     runs-on: ubuntu-24.04-arm
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8  # v5.0.0
+      - uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830  # v4.3.0
+        with:
+          path: |
+            ~/.cache/org.swift.swiftpm
+            ~/.swiftpm
+            .build
+          key: ${{ runner.os }}-swiftpm-${{ hashFiles('Package.resolved') }}
+          restore-keys: ${{ runner.os }}-swiftpm-
       - run: ./scripts/ci-install-swift.sh
       - run: swift --version
       - run: swift build -c release --triple aarch64-none-none-elf --toolset toolset.json