Fix build with gccgo

+ archive: don't convert syscall.Timespec to unix.Timespec
  archive/tar_unix.go:179:76: error: invalid type conversion (cannot use type syscall.Timespec as type unix.Timespec)
    179 |  timespec := []unix.Timespec{unix.Timespec(fs.StatAtime(st)), unix.Timespec(fs.StatMtime(st))}
+ gccgo has no plugin support
  https://github.com/golang/go/issues/36403
+ update github.com/containerd/continuity
  to include same fix for Timespec

Signed-off-by: Shengjing Zhu <zhsj@debian.org>

archive/tar_unix.go

@@ -176,7 +176,10 @@ func copyDirInfo(fi os.FileInfo, path string) error {
 		return errors.Wrapf(err, "failed to chmod %s", path)
 	}
 
-	timespec := []unix.Timespec{unix.Timespec(fs.StatAtime(st)), unix.Timespec(fs.StatMtime(st))}
+	timespec := []unix.Timespec{
+		unix.NsecToTimespec(syscall.TimespecToNsec(fs.StatAtime(st))),
+		unix.NsecToTimespec(syscall.TimespecToNsec(fs.StatMtime(st))),
+	}
 	if err := unix.UtimesNanoAt(unix.AT_FDCWD, path, timespec, unix.AT_SYMLINK_NOFOLLOW); err != nil {
 		return errors.Wrapf(err, "failed to utime %s", path)
 	}