Fix multi-jdk testing setup (#1086)

This changes the Gradle build to always create multi-jdk tasks,
and instead use the `enabled` property to determine whether the task
is actually ran or not.

The has the following benefits:
* IntelliJ and other tools understand the task execution graph (e.g. testJdk20 shows up as a task)
* JDK-specific Gradle configurations always exist, so `updateDependencyLocks` is consistent.

pkl-cli/pkl-cli.gradle.kts

@@ -99,12 +99,7 @@ val testJavaExecutable by
   }
 
 // Setup `testJavaExecutable` tasks for multi-JDK testing.
-val testJavaExecutableOnOtherJdks =
-  if (buildInfo.multiJdkTesting) {
-    buildInfo.multiJdkTestingWith(testJavaExecutable)
-  } else {
-    emptyList()
-  }
+val testJavaExecutableOnOtherJdks = buildInfo.multiJdkTestingWith(testJavaExecutable)
 
 // Prepare a run of the fat JAR, optionally with a specific Java launcher.
 private fun setupJavaExecutableRun(