add --pure option to dev-shell

[mkenigs]

Fixes #2813

If --pure is passed to dev-shell, call execvpe with only the variables in keepVars

[edolstra]

--pure is a confusing name because there is already an --impure flag which is about evaluation. Maybe use --ignore-environment / -i for consistency with nix run and env?

[mkenigs]

Changed to ignore-environment

[edolstra]

Closing, superseded by #3191.