Skip expensive repr() in logging call when not needed (#16350)

We were spending quite a lot of time in this function when running
tests, based on profiling.

Author: JukkaL

mypy/build.py

@@ -255,7 +255,8 @@ def _build(
         stdout=stdout,
         stderr=stderr,
     )
-    manager.trace(repr(options))
+    if manager.verbosity() >= 2:
+        manager.trace(repr(options))
 
     reset_global_state()
     try: