install-nix-from-closure.sh: Use https channel if possible

scripts/install-nix-from-closure.sh

@@ -73,8 +73,12 @@ fi
 
 # Subscribe the user to the Nixpkgs channel and fetch it.
 if ! $nix/bin/nix-channel --list | grep -q "^nixpkgs "; then
+    if [ -n "$SSL_CERT_FILE" ]; then
+        $nix/bin/nix-channel --add https://nixos.org/channels/nixpkgs-unstable
+    else
         $nix/bin/nix-channel --add http://nixos.org/channels/nixpkgs-unstable
     fi
+fi
 $nix/bin/nix-channel --update nixpkgs
 
 # Install an SSL certificate bundle.