Fix cabal pkg-config performance issue (input-output-hk#2178)

The dummy version of `pkg-config` we provide to cabal-install-solver so that it can make an appropriate plan includes an extra blank line at the end of it's output.

In more recent versions of cabal-install-solver the following change has been made:

haskell/cabal@0b34b4e

This change now filters the `null` value from the list of package names.  Then when the versions are looked up the output is not the expected length (because there is also a blank line at the end of the version list).

This change updates the dummy pkg-config script so that it does not include the extra blank lines at the end of the output.

overlays/cabal-pkg-config.nix

@@ -56,17 +56,15 @@ final: prev:
         ${final.pkgs.lib.concatStrings (map (name: ''
           ${name}
         '') (__attrNames pkgconfigPkgs))
-         }
-        EOF2
+         }EOF2
         elif [[ "\$1" == "--modversion" ]]; then
           OUTPUT=\$(mktemp)
           ERROR=\$(mktemp)
         cat <<EOF2
         ${final.pkgs.lib.concatStrings (map (p: ''
           ${getVersion (builtins.head p)}
         '') (__attrValues pkgconfigPkgs))
-        }
-        EOF2
+        }EOF2
         else
           $out/bin/${targetPrefix}${baseBinName}-wrapped "\$@"
         fi