Allow installing from brew cask?

[dstansby]

Currently one can do

jobs:
- template: run-tox-env.yml@OpenAstronomy
  parameters:
    libraries:
      brew:
        - gfortran

But in this example installing gfortran doesn't work, because it needs to install from cask (ie. brew cask install gfortran). It would be nice if there was an option to install packages using brew cask.