feat(nix): open up support to more architectures

This means that the flake can now be built for the following
architectures:

- aarch64-linux
- armv6l-linux
- armv7l-linux
- i686-linux
- powerpc64le-linux
- riscv64-linux
- x86_64-linux

It doesn't mean that it's necessarily going to work perfectly on all of
these architecture and binary cache support is not available for a lot
of them either. But at least it let's you build for them if you want to.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

flake.nix

@@ -34,16 +34,13 @@
       crane,
       ...
     }:
+    let
+      systems = nixpkgs.lib.filter (s: nixpkgs.lib.hasSuffix "-linux" s) nixpkgs.lib.systems.flakeExposed;
+    in
     flake-parts.lib.mkFlake { inherit inputs; } {
       imports = [ inputs.treefmt-nix.flakeModule ];
 
-      # can be extended, but these have proper binary cache support in nixpkgs
-      # as of writing.
-      systems = [
-        "aarch64-linux"
-        "x86_64-linux"
-        "i686-linux"
-      ];
+      inherit systems;
 
       perSystem =
         {