Evaluate on platforms the packages are buildable before scheduling the builds

[Ma27]

An example for the problem is the following PR: https://github.com/NixOS/nixpkgs/pull/59449/checks?check_run_id=101239188

It seems as the darwin machine was quite busy this morning, so it took a while until avidemux was evaluated on Darwin. When the build was started, it didn't build as avidemux isn't supported on Darwin.

I have to admit that I forgot to check the meta section before (then I would've merged right away), but I still think that in terms of UX it's better to only schedule builds on supported platforms (or to abort those immediately rather than queuing those and wait for a free build slot on nix.ci).

As I'm not such an experienced Rust developer, I can't promise that I'll provide a fix in the next time unfortunately, so if anybody else wants to take over, fell free :)