Output lint XML results to a file

Summary:
Ref T8332. I find it really odd that I need to run arc lint --everything --output xml > checkstyle.xml and feel that it would be much more intuitive to just run arc lint --everything --output xml and have the output written to checkstyle.xml.

To provide context, we are running arc lint --everything in a CI job and parsing the results.

Test Plan: Ran arc lint --everything --output xml and saw the output written to file.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8332

Differential Revision: https://secure.phabricator.com/D13570