release-lib: Avoid filtering twice
release-lib: Avoid filtering twice

We were using `supportedMatches` both when getting the list of
platforms, and before `genAttrs` the derivation for each. Now we just
filter the second time,

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs