Force C locale on Ubuntu 16.04 builders

For an unknown reason, newlocale fails for "en_US.UTF-8" occasionally on Ubuntu 16.04. The workaround is to explicitly set LANG=C which does not require glibc to load files since it is builtin to glibc.

This surfaces as

 Failed global initialization: BadValue: Invalid or no user locale set. Please ensure LANG and/or LC_* environment variables are set correctly

because "std::locale::facet::_S_create_c_locale" is throwing a std::runtime_error.