fix: Add toolchains.xml to codebuild agent for JDK 8 resolution

The maven-toolchains-plugin requires JDK 8 to be declared in
~/.m2/toolchains.xml. The smoke test Docker agent has Corretto 8
installed but was missing this file, causing the build to fail.

Author: fabisev

aws-lambda-java-runtime-interface-client/test/integration/codebuild-local/Dockerfile.agent

@@ -11,4 +11,8 @@ COPY --from=docker/buildx-bin:latest /buildx /usr/libexec/docker/cli-plugins/doc
 ENV PATH="$PATH:/apache-maven/bin"
 RUN mkdir /apache-maven && \
     curl https://archive.apache.org/dist/maven/maven-3/3.8.7/binaries/apache-maven-3.8.7-bin.tar.gz | \
-    tar -xz -C /apache-maven --strip-components 1
+    tar -xz -C /apache-maven --strip-components 1
+
+# Declare JDK 8 in toolchains.xml so maven-toolchains-plugin can resolve it
+RUN mkdir -p /root/.m2 && \
+    printf '<?xml version="1.0" encoding="UTF-8"?>\n<toolchains>\n  <toolchain>\n    <type>jdk</type>\n    <provides>\n      <version>8</version>\n    </provides>\n    <configuration>\n      <jdkHome>/usr/lib/jvm/java-1.8.0-amazon-corretto</jdkHome>\n    </configuration>\n  </toolchain>\n</toolchains>\n' > /root/.m2/toolchains.xml