Fix locale issue when forwarded from host environment

Upon first SSHing into his instance, Želko was greeted by a warning about his
locale being invalid, presumably because his SSH client forwards his LOCAL and
LC* environment. I hadn't ran into this before, probably because I've only
tested w/an en-US host. This change silences the warning and coerces an
en_US.UTF-8 locale.

Bug: 46920
Change-Id: Ie5a7c2cb04ec21ba4cfcc8c6a4b23cf0d0db3792