Julia support for v5

[helendduncan]

✅ Checklist

• I have searched open and closed issues for duplicates.
• This is a request for a new feature in the Data Safe Haven or an upgrade to an existing feature.
• The feature is still missing in the latest version.
• I have read through the documentation.
• This isn't an open-ended question (open a discussion if it is).

🍓 Suggested change

Currently Julia isn't supported for dsh v5 as there is no package on the Ubuntu repos or on snapcraft

🚂 How could this be done?

From slack thread here

we previously used this (https://pypi.org/project/jill/) to install Julia. Shouldn't be too hard to ansible-ify it if we need to.