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

@@ -25,7 +25,7 @@ jobs:
     runs-on: ${{ matrix.runs-on }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: builtin:checkout
 
       - name: Install Rust stable
         uses: dtolnay/rust-toolchain@stable
@@ -37,11 +37,16 @@ jobs:
           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
+          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
-          fi
           sccache --version
 
       - name: Ensure build deps (libpipewire + clang for bindgen, C++ for ssstretch)
@@ -120,7 +125,7 @@ jobs:
     runs-on: aarch64
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: builtin:checkout
 
       - name: Compute checksums and update PKGBUILD
         run: |