Adds explicit "systems" input to flake.nix

* Enables building on supported architectures.
* Prevents github from breaking due to unsupported systems by limiting
  the nix flake check to the current system architecture.

Author: choener

.github/workflows/ci-nix.yaml

@@ -55,6 +55,6 @@ jobs:
         nix build --accept-flake-config .#cip-143-cli
         nix build --accept-flake-config .#regulated-stablecoin-cli
     - name: nix flake check
-      run: nix flake --accept-flake-config check
+      run: nix flake check --accept-flake-config --override-input systems github:nix-systems/$(nix eval --impure --raw --expr 'builtins.currentSystem')
     - name: Check aiken
-      run: nix develop --accept-flake-config --command bash -c "cd src/examples/aiken/aiken && aiken build && aiken check"
+      run: nix develop --accept-flake-config --command bash -c "cd src/examples/aiken/aiken && aiken build && aiken check"

flake.lock

@@ -43,13 +43,15 @@
       url = "github:aiken-lang/aiken/v1.1.16";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+
+    systems.url = "github:nix-systems/default";
   };
 
-  outputs = inputs: inputs.iogx.lib.mkFlake {
+  outputs = inputs@{systems, ...}: inputs.iogx.lib.mkFlake {
     inherit inputs;
     repoRoot = ./.;
     outputs = import ./nix/outputs.nix;
-    systems = [ "x86_64-linux" ]; # "x86_64-darwin" ];
+    systems = (import systems);
   };
 
   nixConfig = {