make ghc version available to the test app below

flake.nix

@@ -18,10 +18,11 @@
   in
     ulib.eachSystem ["x86_64-linux" "aarch64-darwin"] (system: let
       pkgs = nixpkgs.legacyPackages.${system};
+      ghcVersion = "ghc8107";
       hslib = hs-flake-utils.lib {
         inherit pkgs;
         src = ./.;
-        compilerVersion = "ghc8107";
+        compilerVersion = ghcVersion;
         packageName = "tahoe-capabilities";
       };
     in {