linter: set MinimumVariableNameLength for readability-identifier-length

.clang-tidy

@@ -35,3 +35,5 @@ CheckOptions:
       value: true
     - key: modernize-avoid-c-arrays.AllowStringArrays
       value: true
+    - key: readability-identifier-length.MinimumVariableNameLength
+      value: 2