prevent doing recursive chroots, by unsetting NIX_ROOT in the scripts.

scripts/download-using-manifests.pl.in

@@ -9,6 +9,7 @@ my $logFile = "@localstatedir@/log/nix/downloads";
 
 open LOGFILE, ">>$logFile" or die "cannot open log file $logFile";
 
+delete $ENV{"NIX_ROOT"};
 
 # Create a temporary directory.
 my $tmpDir;

scripts/nix-prefetch-url.in

@@ -3,6 +3,9 @@
 url=$1
 expHash=$2
 
+# to prevent doing more than 1 chroot
+unset NIX_ROOT
+
 hashType=$NIX_HASH_ALGO
 if test -z "$hashType"; then
     hashType=md5