Add devShell for faster feedback

This adds a devShell which unlike runHydra doesn't start hydra automatically and doesn't receive hydra as build input. It is better suited for interactive development cycles:

$ nix-shell -A devShell
$ ./bootstrap
$ configurePhase
$ make
$ # hack hack hack
$ foreman start
  # test test test
  <C-c>
$ # hack hack hack

Created by  Andreas Rammhold  on May 13, 2020
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