Use Isabelle2024 container for CI

[gauravpartha]

No description provided.

[gauravpartha]

Closing this PR since I prefer having an approach that is independent from a specific container with Isabelle installed. One reason is that the official Isabelle container makarius/isabelle:Isabelle2024 does not include git, which the checkout action seems to require if submodules are present. Installing git on every run is an option (as done in this PR), but I prefer a solution, which instead works on a container with git already installed, since I want a solution that is easily reusable across Isabelle repositories without such additional dependency installing. Another option would be to set up our own container installing Isabelle and git, but then we would have to update that container for every new Isabelle version (not much work, but still extra work).

Instead I merged pull request #6, which uses a separate GitHub action to download and cache Isabelle (the Isabelle version can be provided via a parameter to this action).