Set GOOS and GOARCH fix.

Was unable to set GOOS and GOARCH options using this:
```js
go: {
        options: {
                GOPATH: [ "D:/go" ],
                GOOS: 'linux',
                GOARCH: 'amd64'
        },
        ...
}
```
``` grunt.option('GOOS')``` and ``` grunt.option('GOARCH')``` were returning ``` undefined ```
Changing these calls to ```gruntTaskOpts["GOOS"]``` and ```gruntTaskOpts["GOARCH"]``` solved the problem for me.