Ensure pkg-config can find the nix-supplied zlib package

.github/workflows/test.yml

@@ -110,6 +110,11 @@ jobs:
           esac
           echo "GHC_NIXPKGS=${GHC_NIXPKGS}" >> $GITHUB_ENV
           echo NS="nix shell ${GHC_NIXPKGS}#cabal-install ${GHC_NIXPKGS}#${GHC} ${GHC_NIXPKGS}#gmp ${GHC_NIXPKGS}#zlib ${GHC_NIXPKGS}#zlib.dev" >> $GITHUB_ENV
+          # In a normal nix derivation, there is a pkgconfig hook that automatically
+          # handles this, but since we are just using a nix shell this must be setup
+          # manually so that if the haskell zlib package is built, it finds the right
+          # zlib packge config.
+          echo PKG_CONFIG_PATH=$(nix eval --raw ${GHC_NIXPKGS}#zlib.dev)/lib/pkgconfig >> $GITHUB_ENV
 
       - name: Cabal update
         shell: bash