Add Prebuilt-MPR before installing dependencies

I'm wondering if it would be a good idea to have [Prebuilt-MPR set up](https://docs.makedeb.org/prebuilt-mpr/getting-started/#setting-up-the-repository) in CI? I'm trying to use [`just`](https://github.com/casey/just) which is [only available through `apt` with Prebuild-MPR](https://github.com/casey/just?tab=readme-ov-file#packages).

Other proposed solutions such as #872 might alleviate this need, but right now I can't see a way of working around this.

Another option would be to have a thing like [`taiki-e/install-action`](https://github.com/taiki-e/install-action) to install binaries from in a platform independent way.

One thing that would solve this for me now, is to manually edit `release.yml` but then I need to add `allow-dirty`, which I would rather not.

**Background:** I'm using `just` as part of my [`build.rs`](https://github.com/team-checkr/checkr/blob/d04a78a7324702b1fa06cca4351f97ce48a69d6c/crates/inspectify/build.rs) for building a web frontend which is statically included in the distributed binary using [rust-embed](https://github.com/pyrossh/rust-embed).

Start with release.yml and the dependency-installation workflow, then read the linked Prebuilt-MPR setup documentation. Determine where the repository would be configured before dependencies are installed and whether that permits installing just without allow-dirty. Done means the CI workflow can use that dependency path reliably, or documents why the platform-independent installer is preferred.

Written by the indexing model from the issue text.