torture: Set kvm.sh language to English

Some of the code invoked directly and indirectly from kvm.sh parses
the output of commands.  This parsing assumes English, which can cause
failures if the user has set some other language.  In a few cases,
there are language-independent commands available, but this is not
always the case.  Therefore, as an alternative to polyglot parsing,
this commit sets the LANG environment variable to en_US.UTF-8.

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>