plugin: noopt builder broken by plugin change

The noopt builder is broken by the 03da269 (plugin versioning)

https://build.golang.org/log/857ba03fe3293b11a67a4391292aff2d3b29c28e
```
##### ../misc/cgo/testplugin
2016/11/15 16:27:25 plugin.Open failed: plugin.Open: plugin was built with a different version of package sync/atomic
2016/11/15 16:27:25 Failed: exit status 1
2016/11/15 16:27:25 FAILED
```