`Build Workspace` builds only the current file's folder in a multi-folder workspace

How to repro:
Create a workspace with multiple folders. (folderA, folderB)
Open a go file or not.
Start 'Build Workspace` command (from command palette)
See the Go OUTPUT channel 

```
Starting building the current workspace at /Users/hakim/tmp/folderA
/Users/hakim/tmp/folderA>Finished running tool: /usr/local/go/bin/go build github.com/user/repro/folderA
```