Fix cabal-docspec in Hydra

An extra version of `text` is in the plan.json as a dependency of a
`custom-setup` or `build-tool-depends`. Using `exactDeps = true` prevent those
from getting into the plan.json.

Co-authored-by: Hamish Mackenzie <Hamish.K.Mackenzie@gmail.com>

Author: jasagredo

nix/haskell.nix

@@ -89,9 +89,13 @@ in
     nativeBuildInputs = [
       final.fd
       final.cabal-docspec
-      (hsPkgs.ghcWithPackages
-        (ps: [ ps.latex-svg-image ] ++ lib.filter (p: p ? components.library)
-          (lib.attrValues (haskell-nix.haskellLib.selectProjectPackages ps))))
+      (hsPkgs.shellFor {
+        withHoogle = false;
+        exactDeps = true;
+        packages = _: [ ];
+        additional = (ps: [ ps.latex-svg-image ] ++ lib.filter (p: p ? components.library)
+          (lib.attrValues (haskell-nix.haskellLib.selectProjectPackages ps)));
+      }).ghc
       final.texliveFull
     ];