ci: use container images instead of installing Rust at runtime

Switch to pre-built container images to avoid installing Rust
on every CI run:
- build-linux-x86_64: rust:latest (add x86_64-unknown-linux-musl target)
- build-linux-aarch64: messense/rust-musl-cross:aarch64-musl
  (cross-compiler pre-installed, no Docker-in-Docker needed)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitea/workflows/release.yml

@@ -8,16 +8,13 @@ on:
 jobs:
   build-linux-x86_64:
     runs-on: ubuntu-latest
+    container: rust:latest
     steps:
       - uses: actions/checkout@v4
-      - name: Install Rust stable
-        run: |
-          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable
-          echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
       - name: Add musl target
         run: rustup target add x86_64-unknown-linux-musl
       - name: Install musl tools
-        run: sudo apt-get update && sudo apt-get install -y musl-tools
+        run: apt-get update && apt-get install -y musl-tools
       - name: Build
         run: cargo build --release --target x86_64-unknown-linux-musl
       - name: Create archive
@@ -35,16 +32,11 @@ jobs:
 
   build-linux-aarch64:
     runs-on: ubuntu-latest
+    container: messense/rust-musl-cross:aarch64-musl
     steps:
       - uses: actions/checkout@v4
-      - name: Install Rust stable
-        run: |
-          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable
-          echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
-      - name: Install cross
-        run: cargo install cross --locked
       - name: Build
-        run: cross build --release --target aarch64-unknown-linux-musl
+        run: cargo build --release --target aarch64-unknown-linux-musl
       - name: Create archive
         run: |
           VERSION=${GITHUB_REF_NAME#v}