fix: correct global env path, migration join, and test app dirs

- Use DOKKU_LIB_ROOT instead of DOKKU_ROOT in getGlobalFile() to match
  the intended new config path layout
- Add missing comma in filepath.Join for old app ENV migration path
- Revert common_test.go app dirs to DOKKU_ROOT since app detection
  still uses AppRoot()

plugins/config/environment.go

@@ -396,5 +396,5 @@ func getAppFile(appName string) (string, error) {
 }
 
 func getGlobalFile() string {
-	return filepath.Join(common.MustGetEnv("DOKKU_ROOT"), "config", "--global", "ENV")
+	return filepath.Join(common.MustGetEnv("DOKKU_LIB_ROOT"), "config", "--global", "ENV")
 }