Minor: add explicit MutableMap type annotation to workaround KTIJ-21172

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/PropertiesProvider.kt

@@ -562,7 +562,7 @@ internal class PropertiesProvider private constructor(private val project: Proje
         }
 
     private fun propertiesWithPrefix(prefix: String): Map<String, String> {
-        val result = mutableMapOf<String, String>()
+        val result: MutableMap<String, String> = mutableMapOf()
         project.properties.forEach { (name, value) ->
             if (name.startsWith(prefix) && value is String) {
                 result.put(name, value)