src/goEnvironmentStatus: Go version switch doesn't work when there is no workspace

[hyangah]

Currently, if a different Go version is selected through a UI, this extension stores the changed Go version in the workspace profile (``"go.alternateTools": { "go": ... }`. This approach does not work if there is no workspace. (e.g. single file edit mode).

Either disallow version switch or suggest the global setting change in this case.

cc @mcjcloud

[gopherbot]

Change https://golang.org/cl/241439 mentions this issue: test/integration: disable setSelectedGo test

[gopherbot]

Change https://golang.org/cl/242197 mentions this issue: src/goEnvironmentStatus.ts: remove statusbar item for single files

[hyangah]

We've been disallowing the switch when there is no workspace.