[minor] missing option rename fir -> k2

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/BaseGradleIT.kt

@@ -919,7 +919,7 @@ Finished executing task ':$taskName'|
             }
 
             if (options.useFir) {
-                add("-Pkotlin.useFir=true")
+                add("-Pkotlin.useK2=true")
             }
 
             if (options.dryRun) {

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/testbase/BuildOptions.kt

@@ -127,7 +127,7 @@ data class BuildOptions(
         }
 
         if (useFir) {
-            arguments.add("-Pkotlin.useFir=true")
+            arguments.add("-Pkotlin.useK2=true")
         }
 
         if (usePreciseJavaTracking != null) {