Only build attributes not found in any cache.

This would speed up CI builds significantly and save bandwidth. Currently the cachix-action will pull built packages: https://github.com/Mic92/nur-packages/runs/589727774 In the example above there were 500 MB pulled without a single package built.

This is how it can be implemented:

1. Get all packages: nix-env -f ./non-broken.nix -qaP \* --out-path --xml --meta > packages.xml
2. Get all build hashes: grep -oP '(?<=/nix/store/)[^-]*' < packages.xml (obviously this should be done by proper xml parsing)
3. Only build those attributes where curl -I https://<binary-cache>/<hash>.narinfo does not return a 404

[Mic92]

https://github.com/Mic92/nix-build-uncached it is. Even if your flag lands in Nix it is still going to take a while until it ends in a released version. Until than nix-build-cached will pay the builds.

[zimbatm]

nice! I have a name suggestion: nix-build-uncached. I started adding a nix-build --uncached attribute to Nix itself but this is still at a very early stage.