update_test_checks.py: pick up --tool from UTC_ARGS

It's not clear to me how to write a test for this. The tests run in an
environment where the tools may not be in PATH, and so the existing
custom-tool.test needs to use --tool-binary when invoking
update_test_checks.py. But it can't do so without also using --tool.

This change does fix a problem though with using
update_any_test_checks.py in an environment where the tools *are*
available in PATH.

Differential Revision: https://reviews.llvm.org/D142450