Allow setting GOOS via the Go environment toggle in the UI

[jtsylve]

I really like the current feature of the plugin that lets me switch between Go versions by clicking the button on the bottom toolbar. There are times in which I am developing packages that target specific operating systems, and it would be nice to be able to quickly switch among the systems that I am targeting.

I can currently accomplish these by changing the workspace config and changing the GOOS environment variable in go.toolsEnvVars, but this is a bit tedious when switching back and forth.

Could you please add the ability to set the target OS via the GUI in a convenient way (similar to how I can switch between Go versions)?