Generated virtualenv should be removed when using `ninja clean`

Ninja generator fails with the following error:

ninja clean
...
[1/1] Cleaning all built files...
FAILED: clean 
/usr/bin/ninja  -t clean 
Cleaning... ninja: error: remove(_deps/genopts-build/venv): Directory not empty
ninja: error: remove(/home/amiranda/var/projects/scord/repo/build/_deps/genopts-build/venv): Directory not empty
0 files.
ninja: build stopped: subcommand failed.