Execute subcommands using the "C" locale.

A while back, we changed rbtools to exec the various command-line tools it uses
using the en_US locale (so that we wouldn't get diffs with localized strings in
them). This is nice, but annoying when users don't have the en_US locale on
their system. This change updates things to use the "C" locale instead.

Testing done: Ran unit tests.

Fixes bug 3253.

Reviewed at https://reviews.reviewboard.org/r/5498/