Try #1649:

Author: iohk-bors[bot]

default.nix

@@ -1,10 +1,11 @@
 {...}@args:
 
 let
+  pin = (__fromJSON (__readFile ./flake.lock)).nodes.nixpkgs-2205.locked;
   nixpkgsSrc =
     builtins.fetchTarball {
-      url = "https://github.com/NixOS/nixpkgs/archive/110a2c9ebbf5d4a94486854f18a37a938cfacbbb.tar.gz";
-      sha256 = "sha256-leWXLchbAbqOlLT6tju631G40SzQWPqaAXQG3zH1Imw=";
+      url = "https://github.com/NixOS/nixpkgs/archive/${pin.rev}.tar.gz";
+      sha256 = pin.narHash;
     };
   pkgs = args.pkgs or (import nixpkgsSrc {});
   flake-compat =

test/tests.sh

@@ -39,6 +39,8 @@ fi
 
 if [ "$TESTS" == "unit-tests" ] || [ "$TESTS" == "all" ]; then
   printf "*** Running the unit tests... " >& 2
+  # Running nix build first avoids `error: path '/nix/store/X-hackage-to-nix-ghcjs-overlay.drv' is not valid`
+  nix-build ./default.nix --argstr compiler-nix-name $GHC -A unit.tests
   res=$(nix-instantiate --eval --json --strict ./default.nix --argstr compiler-nix-name $GHC -A unit.tests)
   num_failed=$(jq length <<< "$res")
   if [ $num_failed -eq 0 ]; then