fix: build mode when using a flutter flavor (#51)

plugin.gradle: Use variant.buildType.name instead of variant.name.
The latter is equal to flavor + buildMode while the former is just
buildMode

It fixes this issue
fzyzcjy/flutter_rust_bridge#1606

gradle/plugin.gradle

@@ -141,7 +141,7 @@ class CargoKitPlugin implements Plugin<Project> {
                 }
 
                 def task = project.tasks.create(taskName, CargoKitBuildTask.class) {
-                    buildMode = variant.name
+                    buildMode = variant.buildType.name
                     buildDir = cargoBuildDir
                     outputDir = cargoOutputDir
                     ndkVersion = plugin.project.android.ndkVersion