mypy: Move cache-dir to config file.

mypy.ini

@@ -10,6 +10,7 @@ no_implicit_optional = True
 
 scripts_are_modules = True
 show_traceback = True
+cache_dir = var/mypy-cache
 
 warn_no_return = True
 

tools/run-mypy

@@ -83,7 +83,7 @@ if not python_files and not pyi_files:
     print("There are no files to run mypy on.")
     sys.exit(0)
 
-extra_args = ["--cache-dir=var/mypy-cache"]
+extra_args = []
 if args.linecoverage_report:
     extra_args.append("--linecoverage-report")
     extra_args.append("var/linecoverage-report")