Add CI support

[xaltsc]

I'm trying to address the issues in #526.

So far, I've got a script that updates nix's cef-binary to match the version used by the rust crate cef.

Blocked by #527

Closes #526

[xaltsc]

Please verify the files .github/workflows/cef-nix-sync.yml and renovate.json.

This PR is untested btw, I just know the _update-cef.sh script works on my machine; I have no idea how to test it otherwise.

[xaltsc]

For the record: the standard ways (i.e. Mic92/nix-update and the like) of updating the updated cef-binary package don't work because I couldn't find a way to override the filename attribute this derivation contains (which still points to the original path in nixpkgs).

This is fixed #527

[xaltsc]

There's also a way to run nix flake update using GitHub workflows instead of references, cf https://github.com/xaltsc/jellyfin-desktop/blob/main/.github/workflows/update-flake-lock.yml .

I don't know which one is preferable.

[xaltsc]

So, besides rebasing on the latest version of #527, I've improved the workflows and added one that updates the flake. The two workflows are tested (except for the hook on renovate's PRs about updating CEF) as you can see on my own fork https://github.com/xaltsc/jellyfin-desktop (although I made them before editing this PR).

There are two things that are supposed to update the flake's inputs: the workflow in the last commit, which is tested, and the renovate thing, which ... exists. You probably don't want both, I'm not able to decide which is best, so do tell me which one you want and I'll remove one of the commits.

Except for the points mentioned above, I consider this PR complete.

[xaltsc]

So I added this workflow https://github.com/xaltsc/jellyfin-desktop/blob/flake-ci/.github/workflows/cef-nix-sync-in-pr.yml which should update nix's cef-binary within the same PR as Renovate's ones instead of in another.

This makes sense because, in the meantime between the update of rust cef and that of nix cef-binary the package is broken.

Again, I have no idea what I'm doing, so quintuple check what I've done.

Also, this makes the other workflow obsolete, but, thus far, I've kept it as a working example before review.

[xaltsc]

I removed the old workflow that opened a PR to update nix cef. The new one is much better and works, cf: xaltsc#6
https://github.com/xaltsc/jellyfin-desktop/actions/runs/28466217566