Do not attempt to kill stopped exec processes

While #6204 addressed the stopped state for handling signals in the main
process, it did not update exec processes in the same way. This change
mirrors that adjustment for exec processes.

pkg/shim/proc/exec_state.go

@@ -151,8 +151,8 @@ func (s *execStoppedState) Delete(ctx context.Context) error {
 	return nil
 }
 
-func (s *execStoppedState) Kill(ctx context.Context, sig uint32, all bool) error {
-	return s.p.kill(ctx, sig, all)
+func (s *execStoppedState) Kill(_ context.Context, sig uint32, _ bool) error {
+	return handleStoppedKill(sig)
 }
 
 func (s *execStoppedState) SetExited(int) {