Use cgroups.AddProc() for cgroups v1

All occurrences only passed a PID, so we can use this utility to make
the code more symmetrical with their cgroups v2 counterparts.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

runtime/v1/shim/client/client_linux.go

@@ -35,9 +35,7 @@ func setCgroup(cgroupPath string, cmd *exec.Cmd) error {
 	if err != nil {
 		return fmt.Errorf("failed to load cgroup %s: %w", cgroupPath, err)
 	}
-	if err := cg.Add(cgroups.Process{
-		Pid: cmd.Process.Pid,
-	}); err != nil {
+	if err := cg.AddProc(uint64(cmd.Process.Pid)); err != nil {
 		return fmt.Errorf("failed to join cgroup %s: %w", cgroupPath, err)
 	}
 	return nil