Can't generate matrix when flake supports systems not in platforms map

[purcell]

Hi! I have a flake which supports aarch64-darwin, but GitHub has no runners on that platform. Generating the matrix fails when there's no os found for a system discovered among the flake outputs.

The culprit is the following code, and I'll try to make a PR if I get a few moments:

https://github.com/nix-community/nix-github-actions/blob/bfeb681177b5128d061ebbef7ded30bc21a3f135/default.nix#L30

I guess the best solution would be to skip unsupported platforms but perhaps print a warning.

/cc @akirak