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>

plugin/plugin_go18.go

@@ -1,4 +1,4 @@
-// +build go1.8,!windows,amd64,!static_build
+// +build go1.8,!windows,amd64,!static_build,!gccgo
 
 /*
    Copyright The containerd Authors.

plugin/plugin_other.go

@@ -1,4 +1,4 @@
-// +build !go1.8 windows !amd64 static_build
+// +build !go1.8 windows !amd64 static_build gccgo
 
 /*
    Copyright The containerd Authors.