Use explicitly-generated cabal2nix file

For some reason, relying on `callCabal2nix` in the all-compilers overlay
wasn't working when I tried to depend on our modified `scrypt` package
for our own use - something about the resulting derivation was making
Nix unhappy. However, this manually-generated Nix derivation *does*
work, and is functionally identical, so let's use it. The only annoyance
will be remembering to re-generate the file when we change the `.cabal`
file.

This also avoids a use of IFD, which flakes don't tend to like.

Author: ivanbakel

Diff for: cabal2nix-scrypt.nix

@@ -0,0 +1,21 @@
+{ mkDerivation, base, base64-bytestring, bytestring, c2hs, entropy
+, HUnit, lib, QuickCheck, scrypt-kdf, test-framework
+, test-framework-hunit, test-framework-quickcheck2
+}:
+mkDerivation {
+  pname = "scrypt";
+  version = "0.5.0";
+  src = ./.;
+  libraryHaskellDepends = [
+    base base64-bytestring bytestring entropy
+  ];
+  librarySystemDepends = [ scrypt-kdf ];
+  libraryToolDepends = [ c2hs ];
+  testHaskellDepends = [
+    base bytestring HUnit QuickCheck test-framework
+    test-framework-hunit test-framework-quickcheck2
+  ];
+  homepage = "http://github.com/informatikr/scrypt";
+  description = "Stronger password hashing via sequential memory-hard functions";
+  license = lib.licenses.bsd3;
+}

Diff for: flake.nix

@@ -15,7 +15,7 @@
         haskell = prev.haskell // {
           packageOverrides = final.lib.composeExtensions (prev.haskell.packageOverrides or (_: _: {}))
             (hsFinal: hsPrev: {
-              scrypt = hsFinal.callCabal2nix "scrypt" ./.
+              scrypt = hsFinal.callPackage (import ./cabal2nix-scrypt.nix)
                 { scrypt-kdf = prev.scrypt.dev; };
             });
         };