Drop use of sudo when tee'ing nix.conf

sudo has stuck around since the early days of this repo when /etc/nix/nix.conf
was being created/mutated and required `sudo`. This was changed to a different/local
file where sudo is no longer needed in commit 642cfa0200.

lib/install-nix.sh

@@ -8,7 +8,7 @@ fi
 
 # Configure Nix
 add_config() {
-  echo "$1" | sudo tee -a /tmp/nix.conf >/dev/null
+  echo "$1" | tee -a /tmp/nix.conf >/dev/null
 }
 # Set jobs to number of cores
 add_config "max-jobs = auto"