Add toolchain to gradle config file

This could probably be improved, but it looks to me like it might
be needed to persuade the JDT Language Server (used by Eclipse and
VSCode) to build the project cleanly.

tests/tck-build-logic/src/main/groovy/org.graalvm.internal.tck.gradle

@@ -17,6 +17,13 @@ repositories {
     mavenCentral()
 }
 
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(17)
+        vendor = JvmVendorSpec.GRAAL_VM
+    }
+}
+
 tasks.withType(JavaCompile).configureEach {
     options.release = 11
 }