Enable easy local testing of mulled-build

[daler]

• I have read the guidelines for bioconda recipes.
• This PR adds a new recipe.
• AFAIK, this recipe is directly relevant to the biological sciences (otherwise, please submit to the more general purpose conda-forge channel).
• This PR updates an existing recipe.
• This PR does something else (explain below).

After we moved to CircleCI, it got harder to do local tests with mulled-build due to docker not running nicely from within docker using circleci build. This PR makes some changes to .circleci/setup.sh to make it play nicer with the new driver script bootstrap.py. The driver script creates a custom activate script to activate the new conda installation.

Example usage:

$ ./bootstrap.py /tmp/miniconda
$ source activate ~/.config/bioconda/activate
(BIOCONDA-UTILS) $ bioconda-utils build recipes config.yml --mulled-test --git-range master
(BIOCONDA-UTILS) $ source deactivate

[bgruening]

@daler out of interest what was the error which you are seeing. It seemed to work for me really nicely.

[daler]

@bgruening no error, it's just that I don't think circleci build runs the mulled-build tests (https://github.com/bioconda/bioconda-recipes/blob/master/.circleci/config.yml#L79). So there's not really a good way of locally debugging failures from the mulled-build stage using just the CircleCI client. Also I know folks have complained in the past about messing up their existing root env when installing bioconda-utils, so this tries to take care of all of those points.

[daler]

(making sure a recipe builds correctly, will revert build number once it does)