tue-env switch does not clear TUE variables

[ar13pit]

As tue-env switch is a temporary workspace switching command, a new terminal cannot be opened to clear environment variables. This command however does not clear the existing TUE variables.

How to replicate:

Execute the following commands on the same terminal window:

1. Source an existing tue-env workspace ros-noetic
2. Create a new workspace in a temporary directory tue-env init temp /tmp/temp
3. Switch to the temp workspace tue-env switch temp
4. Echo some TUE variables like, TUE_SYSTEM_DIR, TUE_ROS_DISTRO

[ar13pit]

This happens because of having a default tue-env workspace. I think we should add a warning to tue-env switch when a default workspace has been set.