fix: change Makefile task name

[erral]

Fixes #215

[davisagli]

Changing the name of the target means it will run unless there is a file named venv, which is not the best solution.

I think the real problem is that the requirements-mxdev.txt target doesn't list its dependencies, see my explanation here: #215 (comment)

[stevepiercy]

This is definitely not the correct fix. See my comment in #215 (comment) and a related issue in documentation with a suggested and untested fix plone/documentation#1938.

[ksuess]

I would like to see the prerequisites also in templates/add-ons/backend/{{ cookiecutter.__folder_name }}/Makefile, cause I already had the case that I wanted to test the to be developed backend-add on with another add-on (not part of the repo), just to see that they play well together.

[erral]

@ksuess I have updated the PR with those changes

[ksuess]

Thanks for the adjustments!

[davisagli]

This looks right (or at least, more right 😛) to me

[davisagli]

@erral Thanks for the fix!