buildenv: read propagated-user-env-packages line-by-line

Since 3cb745d, the format of
propagated-user-env-packages has changed and propagated packages have not been
included by buildenv, including in the system environment.

The buildenv builder is modified to read propagated-user-env-packages
line-by-line, instead of expecting all packages on one line.

pkgs/build-support/buildenv/builder.pl

@@ -141,12 +141,11 @@ sub addPkg {
     my $propagatedFN = "$pkgDir/nix-support/propagated-user-env-packages";
     if (-e $propagatedFN) {
         open PROP, "<$propagatedFN" or die;
-        my $propagated = <PROP>;
-        close PROP;
-        my @propagated = split ' ', $propagated;
-        foreach my $p (@propagated) {
+        while (my $p = <PROP>) {
+            chomp $p;
             $postponed{$p} = 1 unless defined $done{$p};
         }
+        close PROP;
     }
 }