Revert "stdenv: Store one package per line in nix-support/propagated-*"

As @oxij points out in [1], this breakage is especially serious because
it changes the contents of built environments without a corresonding
change in their hashes. Also, the revert is easier than I thought.

This reverts commit 3cb745d5a6.

[1]: https://github.com/NixOS/nixpkgs/pull/27427#issuecomment-317293040

pkgs/build-support/setup-hooks/multiple-outputs.sh

@@ -202,7 +202,7 @@ _multioutPropagateDev() {
 
     mkdir -p "${!propagaterOutput}"/nix-support
     for output in $propagatedBuildOutputs; do
-        echo "${!output}" >> "${!propagaterOutput}"/nix-support/$propagatedBuildInputsFile
+        echo -n " ${!output}" >> "${!propagaterOutput}"/nix-support/$propagatedBuildInputsFile
     done
 }