fix(ci): install Maven in each job (runner has no mvn binary)

The runner has Java (via setup-java) but no Maven. Add an
Install Maven step to build-jvm and build-native that downloads
Maven 3.9.6 from Apache to runner.workspace/.maven and adds it
to the PATH via $GITHUB_PATH. Idempotent: the if-guard skips
redownload if the binary already exists on the runner.

.gitea/workflows/ci.yml

@@ -28,6 +28,18 @@ jobs:
           java-version: 21
           cache: maven
 
+      - name: Install Maven
+        env:
+          MAVEN_VERSION: 3.9.6
+          MAVEN_HOME: ${{ runner.workspace }}/.maven
+        run: |
+          mkdir -p "$MAVEN_HOME"
+          if [ ! -x "$MAVEN_HOME/bin/mvn" ]; then
+            curl -fsSL "https://archive.apache.org/dist/maven/maven-3/$MAVEN_VERSION/binaries/apache-maven-$MAVEN_VERSION-bin.tar.gz" | tar -xz -C "$MAVEN_HOME" --strip-components=1
+          fi
+          echo "$MAVEN_HOME/bin" >> "$GITHUB_PATH"
+          mvn --version
+
       - name: Cache Node + npm
         uses: actions/cache@v4
         with:
@@ -60,6 +72,18 @@ jobs:
           java-version: 21
           cache: maven
 
+      - name: Install Maven
+        env:
+          MAVEN_VERSION: 3.9.6
+          MAVEN_HOME: ${{ runner.workspace }}/.maven
+        run: |
+          mkdir -p "$MAVEN_HOME"
+          if [ ! -x "$MAVEN_HOME/bin/mvn" ]; then
+            curl -fsSL "https://archive.apache.org/dist/maven/maven-3/$MAVEN_VERSION/binaries/apache-maven-$MAVEN_VERSION-bin.tar.gz" | tar -xz -C "$MAVEN_HOME" --strip-components=1
+          fi
+          echo "$MAVEN_HOME/bin" >> "$GITHUB_PATH"
+          mvn --version
+
       - name: Set up GraalVM
         uses: graalvm/setup-graalvm@v1
         with: