don't lose environment variables

the resetting was nonesense, Pty appends _environment on top of the
existing environment, so concat the two for potential sandboxing (the
ksandbox class will only apply the diff to system environment anyway).
once the sandboxing was applied reset the actual process environment to
the original variable rather than emptying it, this prevents us from
losing variables when outside a sandbox

BUG: 458591
(cherry picked from commit 252eab85)