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 3cb745d5.

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