nix flake show --all-systems --json --no-write-lock-file fails on armv7a-linux

[grahamc]

Not sure what changed, but nix flake show --all-systems --json --no-write-lock-file is failing to evaluate since just after 07098b4:

      evaluating 'packages.armv7a-linux'...
      error: 'packages.armv7a-linux' is not an attribute set

[oxalica]

It's caused by nixpkgs no longer expose legacyPackages.armv7a-linux anymore.

$ nix eval github:NixOS/nixpkgs#legacyPackages --apply builtins.attrNames
[ "aarch64-darwin" "aarch64-linux" "armv5tel-linux" "armv6l-linux" "armv7l-linux" "i686-linux" "mipsel-linux" "powerpc64le-linux" "riscv64-linux" "x86_64-darwin" "x86_64-linux" ]

Probably we should follow systems in nixpkgs.lib.systems.flakeExposed instead of maintaining our own list, so it can keep in sync with nixpkgs.