ci: no sccache install on the bare aarch64 (klokka) runner; builtin:checkout

Gate the sccache install to the ephemeral x86_64 container; on the bare klokka
runner check-and-fail (the tarball was x86_64-only, so it was a host mutation
and a wrong-arch binary). Switch checkout to v4 builtin:checkout.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -16,14 +16,16 @@ jobs:
           - arch: x86_64
             target: x86_64-unknown-linux-gnu
             runs-on: ubuntu-latest
+            sccache_dir: /sccache
           - arch: aarch64
             target: aarch64-unknown-linux-gnu
             runs-on: aarch64
+            sccache_dir: /var/lib/gitea-runner/sccache
 
     runs-on: ${{ matrix.runs-on }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: builtin:checkout
 
       - name: Install system dependencies
         if: matrix.arch == 'x86_64'
@@ -36,11 +38,26 @@ jobs:
         with:
           targets: ${{ matrix.target }}
 
-      - name: Cache cargo registry
+      # Compiled-crate cache via sccache. x86_64 uses /sccache (a persistent
-        uses: actions/cache@v3
+      # host dir bind-mounted into the job container by the runner config);
-        with:
+      # aarch64 runs on the klokka host and uses a runner-owned dir. Both
-          path: ~/.cargo/registry
+      # persist across releases, unlike the (unreachable) Gitea cache service.
-          key: ${{ runner.os }}-${{ matrix.arch }}-cargo-${{ hashFiles('Cargo.lock') }}
+      - name: Set up sccache
+        run: |
+          echo "RUSTC_WRAPPER=sccache" >> "$GITHUB_ENV"
+          echo "SCCACHE_DIR=${{ matrix.sccache_dir }}" >> "$GITHUB_ENV"
+          mkdir -p "${{ matrix.sccache_dir }}"
+          if command -v sccache >/dev/null 2>&1; then sccache --version; exit 0; fi
+          # Bare runners (aarch64 = klokka host) are pre-provisioned; CI must not
+          # install onto them (and this tarball is x86_64-only). Only the
+          # ephemeral x86_64 container installs; a bare host missing it fails loud.
+          if [ "${{ matrix.arch }}" != x86_64 ]; then
+            echo "::error::sccache missing on the bare ${{ matrix.arch }} runner — provision klokka; CI must not install on bare hosts"; exit 1
+          fi
+          V=0.8.2
+          curl -sSL "https://github.com/mozilla/sccache/releases/download/v${V}/sccache-v${V}-x86_64-unknown-linux-musl.tar.gz" | tar -xz
+          sudo install -m0755 "sccache-v${V}-x86_64-unknown-linux-musl/sccache" /usr/local/bin/sccache
+          sccache --version
 
       - name: Test
         run: cargo test --target ${{ matrix.target }}
@@ -100,7 +117,7 @@ jobs:
     runs-on: aarch64
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: builtin:checkout
 
       - name: Compute checksums and update PKGBUILD
         run: |