flake-info: support platform patterns

[ncfavier]

nixpkgs currently defines three ways of specifying elements of meta.{platforms,badPlatforms}:

1. as a string; this is okay
2. as a pattern, which is an attribute set meant to match a platform's parsed field (or possibly in the future an entire platform, see meta: replace predicates with pattern over elaborated platform nixpkgs#212939). This has apparently been a thing for a long time but wasn't used until about a week ago. For now, just skip them without causing an error. Later, maybe figure out a way to display them properly.
3. as a function platform → bool; this causes nix-env to emit null which is not great but at least we support it fine.

For reference, the isGnu list of patterns looks like this:

[
  { abi = { _type = "abi"; abi = "64"; name = "gnuabi64"; }; }
  { abi = { _type = "abi"; name = "gnu"; }; }
  { abi = { _type = "abi"; float = "soft"; name = "gnueabi"; }; }
  { abi = { _type = "abi"; float = "hard"; name = "gnueabihf"; }; }
  { abi = { _type = "abi"; abi = "elfv1"; name = "gnuabielfv1"; }; }
  { abi = { _type = "abi"; abi = "elfv2"; name = "gnuabielfv2"; }; }
]

so the old System struct we removed in #580 wouldn't have helped here.