Make Kythe run work with Gradle 7 (#1727)

The fix is based on b/240627423. I tested locally and was able to build
with the -PenableCrossReferencing=true flag successfully.

TESTED=run the kythe GCB pipeline locally.

build.gradle

@@ -207,8 +207,8 @@ allprojects {
     gradle.projectsEvaluated {
       tasks.withType(JavaCompile) {
         options.fork = true
-        options.forkOptions.executable =
-            "${project.rootDir}/kythe/extractors/javac-wrapper.sh"
+        options.forkOptions.javaHome =
+          file("${System.env.REAL_JAVA_HOME}")
       }
     }
   }

release/cloudbuild-kythe.yaml

@@ -31,6 +31,10 @@ steps:
   - -c
   - |
     export REAL_JAVAC=$(which javac)
+    export REAL_JAVA_HOME=$${PWD}/java_home
+    cp -r $(dirname $(dirname $(readlink -f $(which javac)))) $${REAL_JAVA_HOME}
+    rm $${REAL_JAVA_HOME}/bin/javac
+    cp ./kythe/extractors/javac-wrapper.sh $${REAL_JAVA_HOME}/bin/javac
     export JAVAC_EXTRACTOR_JAR="$${PWD}/kythe/extractors/javac_extractor.jar"
     export KYTHE_VNAMES="$${PWD}/vnames.json"
     export KYTHE_ROOT_DIRECTORY="$${PWD}"