vm/windows: Save envirenoment before running VM.

Later, when we start the actual builder, we're going to restore those
environment variables. We're using "(set; declare -p)", here, because
the former is just printing _all_ environment variables, even those not
supported, and the latter only lists specifically declared variables,
which also encludes exports.

The "declare -p" command also emits those variables in a format similar
to the "export" command.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>