Maybe run CI only on manual?

Currently we build the image each time we push to master, but it might in fact be counter-productive:

• Most of the time we spend time and ressources rebuilding almost exactly the same image (the notebooks aren't included inside)
• If one day the conda environment breaks (similarly to what happens with macOS 11.3 currently) we would lose the images.

For the second point I intend to mitigate it by providing a tag once our current version is stabilized, but choosing when updating the images would be even better.