Stop forking javac (elastic#30462)

We started forking javac to avoid GC overhead when running builds. Yet,
we do not seem to have this problem anymore and not forking leads to a
substantial speed improvement. This commit stops forking javac.

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -497,10 +497,15 @@ class BuildPlugin implements Plugin<Project> {
         project.afterEvaluate {
             project.tasks.withType(JavaCompile) {
                 final JavaVersion targetCompatibilityVersion = JavaVersion.toVersion(it.targetCompatibility)
-                // we fork because compiling lots of different classes in a shared jvm can eventually trigger GC overhead limitations
-                options.fork = true
-                options.forkOptions.javaHome = new File(project.compilerJavaHome)
-                options.forkOptions.memoryMaximumSize = "512m"
+                final compilerJavaHomeFile = new File(project.compilerJavaHome)
+                // we only fork if the Gradle JDK is not the same as the compiler JDK
+                if (compilerJavaHomeFile.canonicalPath == Jvm.current().javaHome.canonicalPath) {
+                    options.fork = false
+                } else {
+                    options.fork = true
+                    options.forkOptions.javaHome = compilerJavaHomeFile
+                    options.forkOptions.memoryMaximumSize = "512m"
+                }
                 if (targetCompatibilityVersion == JavaVersion.VERSION_1_8) {
                     // compile with compact 3 profile by default
                     // NOTE: this is just a compile time check: does not replace testing with a compact3 JRE