Apply $wgShellLocale in Setup.php

Apply $wgShellLocale in Setup.php

While most of MediaWiki ignores the shell's/C library's locale setting,
there are some things other than shell execs that use it (e.g. the
luasandbox PHP extension).

To provide a consistent environment, set the locale in Setup.php instead
of letting it be changed mid-request depending on whether something else
happened to have called certain functions.

Bug: T107128
Change-Id: I02943803d26d5b1b3ac00ef9216f69cdfa149585