fix: don’t nuke an activated virtual environment

[jenstroeger]

So… calling make nuke usually fails because deactivate can’t be found when running make. That is because deactivate is actually a function sourced by the activate script

 > type -t deactivate 
function

and therefore it’s not available in the subprocess that runs the shell in the Makefile. This change prevents that failure, and instead just aborts make cleanly if the venv is activated.