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/misc/misc.nix

@@ -23,7 +23,7 @@ in
   */
   collection = {list, name} : runCommand "collection-${name}" {} ''
     mkdir -p $out/nix-support
-    printLines ${builtins.toString list} > $out/nix-support/propagated-user-env-packages
+    echo ${builtins.toString list} > $out/nix-support/propagated-user-env-packages
   '';
 
   /* creates a derivation symlinking references C/C++ libs into one include and lib directory called $out/cdt-envs/${name}