Reduce noise in buildbot logs

Set LANG=C also when providing a command to enter_chroot.sh. Should make
buildbot logs quite a bit shorter.

Review URL: http://codereview.chromium.org/521053
diff --git a/enter_chroot.sh b/enter_chroot.sh
index 31734a9..a132cc2 100755
--- a/enter_chroot.sh
+++ b/enter_chroot.sh
@@ -140,7 +140,7 @@
 # the source trunk for scripts that may need to print it (e.g.
 # build_image.sh).
 sudo chroot "$FLAGS_chroot" sudo -i -u $USER $CHROOT_PASSTHRU \
-  EXTERNAL_TRUNK_PATH="${FLAGS_trunk}" "$@"
+  EXTERNAL_TRUNK_PATH="${FLAGS_trunk}" LANG=C "$@"
 
 # Remove trap and explicitly unmount
 trap - EXIT