nixos/tzupdate: Fix ShellCheck issue

Fixes the following warning when setting
`systemd.enableStrictShellChecks = true`:

> In /nix/store/lbzbwz5w0r7rn1m2h3cwa57nfs0xy6dd-unit-script-tzupdate-start/bin/tzupdate-start line 6:
> timedatectl set-timezone $(/nix/store/q9bahab38yvn2lr3r3sdkvlnkg1rkzng-tzupdate-3.1.0/bin/tzupdate --print-only)
>                          ^-- SC2046 (warning): Quote this to prevent word splitting.

nixos/modules/services/misc/tzupdate.nix

@@ -28,7 +28,7 @@ in {
       wants = [ "network-online.target" ];
       after = [ "network-online.target" ];
       script = ''
-        timedatectl set-timezone $(${lib.getExe pkgs.tzupdate} --print-only)
+        timedatectl set-timezone "$(${lib.getExe pkgs.tzupdate} --print-only)"
       '';
 
       serviceConfig = {