CI: Test fslpython installation on new commits

Every time commits are added to fsl/post-install, it will do a test run of fslpython_install.sh script within a docker condainer, and on macOS. The jobs are currently being executed on gitlab runners installed on my old macbook (sitting on my desk at FMRIB), and on a runner managed by Duncan.

The test depends on fslpython_install.sh exiting with a non-zero code when the installation fails.

The scope of this MR has also been expanded to encompass a general clean-up and update of all third-party dependencies that are installed into the fslpython environment.