haskellPackages.hnix: unbreak

The mystery build failure was caused by having the same instance as an
orphan and imported from ref-tf 0.5 (why ever that doesn't warrant a
logged error message…). The solution for this is
https://github.com/haskell-nix/hnix/pull/918, which sadly doesn't apply
cleanly on the hnix 0.12.0.1 tarball. Therefore I've backported the
patch until hnix hopefully gets a new hackage release soon.

pkgs/development/haskell-modules/configuration-common.nix

@@ -197,7 +197,19 @@ self: super: {
   digit = doJailbreak super.digit;
 
   # 2020-06-05: HACK: does not pass own build suite - `dontCheck`
-  hnix = generateOptparseApplicativeCompletion "hnix" (dontCheck super.hnix);
+  hnix = generateOptparseApplicativeCompletion "hnix"
+    (overrideCabal super.hnix (drv: {
+      doCheck = false;
+      prePatch = ''
+        # fix encoding problems when patching
+        ${pkgs.dos2unix}/bin/dos2unix hnix.cabal
+      '' + (drv.prePatch or "");
+      patches = [
+        # support ref-tf in hnix 0.12.0.1, can be removed after
+        # https://github.com/haskell-nix/hnix/pull/918
+        ./patches/hnix-ref-tf-0.5-support.patch
+      ] ++ (drv.patches or []);
+    }));
 
   # Fails for non-obvious reasons while attempting to use doctest.
   search = dontCheck super.search;