Enable Gradle configuration cache (#1646)

Enable the configuration cache feature in Gradle, and adjust various
pieces of build logic that aren't configuration cache compatible.

pkl-config-kotlin/pkl-config-kotlin.gradle.kts

@@ -57,10 +57,11 @@ val generateTestConfigClasses by
 
     classpath = pklCodegenKotlin
     mainClass.set("org.pkl.codegen.kotlin.Main")
+    val codegenSources = fileTree("src/test/resources/codegenPkl")
     argumentProviders.add(
       CommandLineArgumentProvider {
         listOf("--output-dir", outputDir.get().asFile.absolutePath) +
-          fileTree("src/test/resources/codegenPkl").map { it.absolutePath }
+          codegenSources.map { it.absolutePath }
       }
     )
   }