mulled always downloads involucro unless it is in the current working directory or --involucro-path is provided. It fails to detect if involucro is available via PATH since https://github.com/galaxyproject/galaxy-lib/blob/18.5.7/galaxy/tools/deps/mulled/mulled_build.py#L288 does not inspect PATH.
In Python >=3.3 this could be solved by using shutil.which.
For Python 2 compatibility, there are some packages, e.g., backports.shutil_which or whichcraft, that backport the functionality from https://github.com/python/cpython/blob/3.7/Lib/shutil.py#L1109-L1169.