Add LC_ALL=en_US.UTF-8 before running check-man-warnings

It will be fail if LC_ALL was set to non-utf8 locale(e.g, 'C') by
following error.

    col: Invalid or incomplete multibyte or wide character
    man: command exited with status 1: col -b -p -x

Signed-off-by: Yuto KAWAMURA(kawamuray) <kawamuray.dadada@gmail.com>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>