buildEnv: Fix handling of empty list of packages

pkgs/build-support/buildenv/builder.pl

···
 # For each activated package, determine what symlinks to create.
 
 my %symlinks;
-$symlinks{""} = ["", 0]; # create root directory
 
-my %priorities;
+for my $p (@pathsToLink) {
+    $p = "" if $p eq "/";
+    $symlinks{$p} = ["", 0];
+}
 
 sub findFiles;