xds: Allow Gradle to use more memory when building interop

Should fix "Expiring Daemon because JVM heap space is exhausted".

https://github.com/grpc/grpc-java/pull/9269 probably pushed the build
over the edge, but there's been evidence via flakes for a good while
that we've been reaching the limit.

b/238334438

buildscripts/kokoro/psm-security.sh

@@ -23,6 +23,7 @@ readonly BUILD_APP_PATH="interop-testing/build/install/grpc-interop-testing"
 build_java_test_app() {
   echo "Building Java test app"
   cd "${SRC_DIR}"
+  GRADLE_OPTS="-Dorg.gradle.jvmargs='-Xmx1g'" \
   ./gradlew --no-daemon grpc-interop-testing:installDist -x test \
     -PskipCodegen=true -PskipAndroid=true --console=plain
 

buildscripts/kokoro/xds_k8s_lb.sh

@@ -23,6 +23,7 @@ readonly BUILD_APP_PATH="interop-testing/build/install/grpc-interop-testing"
 build_java_test_app() {
   echo "Building Java test app"
   cd "${SRC_DIR}"
+  GRADLE_OPTS="-Dorg.gradle.jvmargs='-Xmx1g'" \
   ./gradlew --no-daemon grpc-interop-testing:installDist -x test \
     -PskipCodegen=true -PskipAndroid=true --console=plain
 

buildscripts/kokoro/xds_url_map.sh

@@ -23,6 +23,7 @@ readonly BUILD_APP_PATH="interop-testing/build/install/grpc-interop-testing"
 build_java_test_app() {
   echo "Building Java test app"
   cd "${SRC_DIR}"
+  GRADLE_OPTS="-Dorg.gradle.jvmargs='-Xmx1g'" \
   ./gradlew --no-daemon grpc-interop-testing:installDist -x test \
     -PskipCodegen=true -PskipAndroid=true --console=plain