Update shell script shebangs

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

lib/scripts/add-host.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/usr/bin/env -S nix shell nixpkgs#ssh-to-age -c bash
 
 set -o errexit
 set -o nounset
@@ -15,7 +15,7 @@ mkdir -p "./hosts/$host/secrets"
 
 ssh-keygen -t ed25519 -f "./hosts/$host/secrets/ssh_host_ed25519_key" -N ""
 
-age_key=$(nix shell nixpkgs#ssh-to-age --command bash -c "cat './hosts/$host/secrets/ssh_host_ed25519_key.pub' | ssh-to-age")
+age_key=$(cat './hosts/$host/secrets/ssh_host_ed25519_key.pub' | ssh-to-age)
 
 find . -type f -name "sops.yaml" | while IFS= read -r sops_file; do
   sed -i "/- hosts:/a\    - &$host $age_key" "$sops_file"