Fix SUREFIRE_JVM_OPT for sample build

Author: cindy-peng

.cloudbuild/samples_build.yaml

@@ -24,7 +24,7 @@ steps:
   - 'KOKORO_GITHUB_PULL_REQUEST_NUMBER=$_PR_NUMBER'
   - 'JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64'
   - 'JAVA=/usr/lib/jvm/java-11-openjdk-amd64'
-  - 'SUREFIRE_JVM_OPT=-Djvm=$/usr/lib/jvm/java-11-openjdk-amd64/bin/java'
+  - 'SUREFIRE_JVM_OPT=-Djvm=/usr/lib/jvm/java-11-openjdk-amd64/bin/java'
 - name: gcr.io/cloud-devrel-public-resources/java8
   entrypoint: echo
   args: [