Try to fix OS X workflow

.github/workflows/nix.yml

@@ -40,4 +40,4 @@ jobs:
  # run: nix-shell --run echo
 
  - continue-on-error: false
- run: nix flake check --allow-import-from-derivation
+ run: nix flake check --allow-import-from-derivation --impure

flake.nix

@@ -11,9 +11,6 @@
  "aarch64-linux"
  "aarch64-darwin"
  ];
- # For `nix flake show` and `nix flake check`
- # Requires --impure, on pure evaluation we default to x86_64-linux
- currentPlatform = builtins.currentSystem or "x86_64-linux";
  in
  flake-utils.lib.eachSystem supportedSystems (system:
  let
@@ -40,8 +37,6 @@
  compiler-nix-name = ghcVersion; # Version of GHC to use
  # Use the corresponding stack configuration as well
  stackYaml = value;
- # For `nix flake show` to work:
- evalSystem = currentPlatform;
  # Tools to include in the development shell
  shell.tools = {
  cabal = "latest";
@@ -61,7 +56,8 @@
  # Disable check for doctests
  packages.BNFC.components.tests.doctests.doCheck = false;
  }];
- };
+ # For `nix flake show --impure` and `nix flake check --impure` to work
+ } // ( if (builtins ? currentSystem) then {evalSystem = builtins.currentSystem;} else {});
  }) supportedGHCStackFile
  )
  ];
@@ -75,7 +71,7 @@
  # defined for all platforms, which can cause check failures (see issue NixOS/nix#6453)
  # For `nix flake check` to work, filter out all hydrajobs that are not for the current platform 
  # Note that on non x86_64-linux platforms --impure is required to pick up the correct platform names
- flake = if system == currentPlatform
+ flake = if system == (builtins.currentSystem or "x86_64-linux")
  then allFlake
  else builtins.removeAttrs allFlake ["hydraJobs"];
  in flake // {