treewide: set shebang for bash scripts

These scripts previously failed when called with syscalls like
`execve` (used by, e.g., Python's `subprocess.run`) that use no default
interpreter for scripts without a shebang.

modules/lightning-pool.nix

@@ -49,7 +49,7 @@ let
       description = "Extra lines appended to the configuration file.";
     };
     cli = mkOption {
-      default = pkgs.writeScriptBin "pool" ''
+      default = pkgs.writers.writeBashBin "pool" ''
         exec ${cfg.package}/bin/pool \
           --rpcserver ${nbLib.addressWithPort cfg.rpcAddress cfg.rpcPort} \
           --network ${network} \