Fix CI: use --break-system-packages for pip, cache zigbuild, keep apt fallback

- Ubuntu 24.04 blocks bare pip install (PEP 668), add --break-system-packages
- Move cache steps before cargo install so zigbuild binary can be cached
- Cache cargo-zigbuild binary separately, skip install if already cached
- Keep apt system cross-linkers as fallback alongside zigbuild

https://claude.ai/code/session_01EcsZDNeWn56wFqryb4Wq7r

.github/workflows/ci.yml

@@ -25,10 +25,6 @@ jobs:
         toolchain: ${{ matrix.rust }}
         targets: x86_64-unknown-linux-musl,aarch64-unknown-linux-musl
         components: rustfmt,clippy
-    - name: Install zig
-      run: pip install ziglang
-    - name: Install cargo-zigbuild
-      run: cargo install cargo-zigbuild
     - name: Cache cargo registry
       uses: actions/cache@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v4
       with:
@@ -44,6 +40,19 @@ jobs:
       with:
         path: target
         key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+    - name: Cache cargo-zigbuild
+      uses: actions/cache@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v4
+      with:
+        path: ~/.cargo/bin/cargo-zigbuild
+        key: ${{ runner.os }}-cargo-zigbuild-${{ matrix.rust }}
+    - name: Install zig and cargo-zigbuild
+      run: |
+        pip install --break-system-packages ziglang
+        command -v cargo-zigbuild || cargo install cargo-zigbuild
+    - name: Install system cross-linkers (fallback)
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y musl-tools gcc-aarch64-linux-gnu gcc-x86-64-linux-gnu
 
     - run: make check-fmt
     - run: make lint