Check expected miniconda.sh hash, redownloads if necessary

This MR introduces a hash check on the expected version of the miniconda installer.

It re-downloads the installer if the hash does not match or if it is not cached.

This solves an issue in which more recent versions of the conda (4.7.x) take very long to bootstrap bob.devtools.

enabled an automatic merge when the pipeline for aac36dd3 succeeds

merged

mentioned in commit dd47cc03