run-phpcs-mw: Specify encoding of source files as utf-8

Port of https://gerrit.wikimedia.org/r/#/c/88743 against JJB macro:

Per
http://pear.php.net/manual/en/package.php.php-codesniffer.advanced-usage.php.
Bug: 55508

Change-Id: I44f42262d709b2b9ad876bdf9fc44fb5e3dfff09