qga: remove explicit environ argument from exec/spawn

Environment is implicitly inherited from the current process "environ"
variable for execl() or g_spawn_sync(), no need to be explicit about it.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220323155743.1585078-31-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>