fix: Block commands from using an invalid workspace name. (#38594)

We're blocking commands from using an invalid workspace name, as this mitigates some risks if the invalid name attempts to cause path traversal. A workspace is selected if its name is in the .terraform/environment file, and something other than Terraform could change the contents of that file. To fix this, we expect users to use Terraform to update that file via Terraform using  the `workspace select` command. This will allow the user to select or create a valid workspace instead, while also removing the bad data from the file without expecting the user to know about that file (it's an implementation detail, not a public API!).