meson: Add nspawn-locale meson option

https://github.com/systemd/systemd/pull/23192 caused breakage in
Arch Linux's build tooling. Let's give users an opt-out aside from
reverting the patch. It's hardly any maintenance work on our side
and gives users an easy way to revert the locale change if needed.

Of course, by default we still pick C.UTF-8 if the option is not
specified.

src/nspawn/nspawn.c

@@ -3488,10 +3488,7 @@ static int inner_child(
         }
 
         if (arg_start_mode != START_BOOT) {
-                /* If we're running a command in the container, let's default to the C.UTF-8 locale as it's
-                 * part of glibc these days and was backported to most distros a long time before it got
-                 * added to upstream glibc. */
-                envp[n_env] = strdup("LANG=C.UTF-8");
+                envp[n_env] = strdup("LANG=" SYSTEMD_NSPAWN_LOCALE);
                 if (!envp[n_env])
                         return log_oom();
                 n_env++;