Reduce the number of systems offered by the flake

This gets rid of evaluation error for armv7a-darwin. It makes no sense
to support this platform in this overlay. According to the discussion
around around
NixOS/nixpkgs#324155 (comment),
it is better to support just the systems exposed by the nixpkgs flake.

Currently, these are:
- aarch64-darwin
- aarch64-linux
- armv6l-linux
- armv7l-linux
- i686-linux
- powerpc64le-linux
- riscv64-linux
- x86_64-darwin
- x86_64-freebsd
- 86_64-linux

flake.nix

@@ -9,7 +9,7 @@
   outputs = { self, nixpkgs, flake-utils }:
     with nixpkgs.lib;
     with flake-utils.lib;
-    eachSystem allSystems (system: let
+    eachSystem systems.flakeExposed (system: let
       pkgs = import nixpkgs {
         inherit system;
         overlays = [ self.overlays.default ];