ci(build): add Rust toolchain and cargo-ndk for native-certgen

Gradle's buildRustCertgen task requires cargo-ndk and Android NDK
targets to cross-compile libcertgen.so. Without these, CI fails on
any commit after 6aba82e which wired the Rust crate into the pipeline.

.github/workflows/build.yml

@@ -45,6 +45,26 @@ jobs:
           java-version: 21
           cache: 'gradle'
 
+      - name: Setup Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          targets: aarch64-linux-android,armv7-linux-androideabi,i686-linux-android,x86_64-linux-android
+
+      - name: Cache Rust artifacts
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            ~/.cargo/bin/cargo-ndk
+            native-certgen/target
+          key: rust-${{ runner.os }}-${{ hashFiles('native-certgen/Cargo.lock') }}
+          restore-keys: |
+            rust-${{ runner.os }}-
+
+      - name: Install cargo-ndk
+        run: command -v cargo-ndk || cargo install cargo-ndk
+
       - name: Set up ccache
         uses: hendrikmuhs/ccache-action@v1.2
         with:
@@ -60,7 +80,6 @@ jobs:
       - name: Build with Gradle
         run: |
           chmod +x ./gradlew
-
           ./gradlew zipRelease zipDebug -Porg.gradle.parallel=true -Porg.gradle.vfs.watch=true -Dorg.gradle.jvmargs=-Xmx2048m
 
       - name: Prepare artifact