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/stdenv/generic/builder.sh

@@ -1,3 +1,4 @@
+export PATH=
 for i in $initialPath; do
     if [ "$i" = / ]; then i=; fi
     PATH=$PATH${PATH:+:}$i/bin
@@ -14,6 +15,4 @@ cat "$setup" >> $out/setup
 # Allow the user to install stdenv using nix-env and get the packages
 # in stdenv.
 mkdir $out/nix-support
-if [ "$propagatedUserEnvPkgs" ]; then
-    printf '%s\n' $propagatedUserEnvPkgs > $out/nix-support/propagated-user-env-packages
-fi
+echo $propagatedUserEnvPkgs > $out/nix-support/propagated-user-env-packages