Only assign options.isFork if needed

This is in preparation for Gradle 9 that will change isFork
to a lazy property (and dynamically rewrite the plugin
bytecode) which will make it immutable at execution time.

With this change, users can manually configure isFork=true
when needed to avoid the error, as the plugin will no longer
try to set isFork to true if it already is.

This is a first step towards fixing #97

src/main/kotlin/net/ltgt/gradle/errorprone/ErrorPronePlugin.kt

@@ -135,7 +135,7 @@ class ErrorPronePlugin @Inject constructor(
                 if (!errorproneOptions.isEnabled.getOrElse(false)) return@doFirst
                 jvmArgumentProvider.compilerVersion?.let {
                     if (it < JavaVersion.VERSION_1_8) throw UnsupportedOperationException(TOO_OLD_TOOLCHAIN_ERROR_MESSAGE)
-                    if (it == JavaVersion.VERSION_1_8 || (it == JavaVersion.current() && CURRENT_JVM_NEEDS_FORKING)) options.isFork = true
+                    if ((it == JavaVersion.VERSION_1_8 || (it == JavaVersion.current() && CURRENT_JVM_NEEDS_FORKING)) && !options.isFork) options.isFork = true
                 }
             }
         }