seems to be here instead

flake.nix

@@ -84,7 +84,7 @@
               export PKG_CONFIG_PATH=${pkgs.lib.makeSearchPath "lib/pkgconfig" [pkgs.zlib.dev]}
 
               find .
-              ghc-pkg check --package-db .cabal/store/ghc-8.10.7/package.db
+              ghc-pkg check --package-db .cabal/ghc-8.10.7/package.db
               cabal update hackage.haskell.org
               cabal run tests
             '';