Consider installing `just` with `apt-get` on Ubuntu 24.04 runners

The action currently use the install-just.sh script committed to this repo to install just on the GitHub CI runners.

The benefit of using the script (which is pretty much a copy of https://github.com/casey/just/blob/master/www/install.sh) is that it supports installation on Windows, MacOS and Linux CI runners. The script downloads just from GitHub and fetches the specified version, which at the time of writing is 1.34.0:
https://github.com/opensafely-core/setup-action/blob/93841f27c28c75bf85b1a0a915e2391a06c6c8a6/action.yml#L44
https://github.com/opensafely-core/setup-action/blob/93841f27c28c75bf85b1a0a915e2391a06c6c8a6/install-just.sh#L32-L33

We previously preferred the use of the installation script to fetch just directly from GitHub over delegating to third parties such as uv or setup-just.

On Ubuntu 24.04 (but not Ubuntu 22.04), just can be installed with apt get. Perhaps installing just via Ubuntu would be more secure than fetching it from https://github.com/casey/just/releases in terms of the likelihood of a vulnerable / compromised release of just ending up in our runners.

We should consider installing just via apt get on Ubuntu 24.04 runners, and only use the install-just.sh script for Windows, MacOS and Ubuntu 22.04 runners. This could potentially reduce the attack surface of our services.

Start with action.yml at line 44 and install-just.sh at lines 32-33 to understand the current version and installation path. Check how the action identifies Ubuntu 24.04 versus other runners, then verify that Ubuntu 24.04 uses apt while Windows, macOS, and Ubuntu 22.04 retain the script; done means the runner-specific installation behavior is correct.