CI: sccache-backed compiled-crate cache (persistent, replaces the dead cache service)

x86_64 uses /sccache (host dir bind-mounted into the job container by the
runner config); aarch64 on the klokka host uses a runner-owned dir. Both
persist across releases via RUSTC_WRAPPER=sccache.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GLUwWE2KmFPzhKaf67tWbx

.gitea/workflows/release.yml

@@ -15,8 +15,10 @@ jobs:
         include:
           - arch: x86_64
             runs-on: ubuntu-latest
+            sccache_dir: /sccache
           - arch: aarch64
             runs-on: aarch64
+            sccache_dir: /var/lib/gitea-runner/sccache
 
     runs-on: ${{ matrix.runs-on }}
 
@@ -28,6 +30,22 @@ jobs:
         with:
           targets: wasm32-unknown-unknown
 
+      # Compiled-crate cache via sccache. x86_64 uses /sccache (a persistent
+      # host dir bind-mounted into the job container by the runner config);
+      # aarch64 runs on the klokka host and uses a runner-owned dir. Both
+      # persist across releases, unlike the (unreachable) Gitea cache service.
+      - 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
+            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: Install cargo-binstall
         run: |
           command -v cargo-binstall || \