Fix meson build on macOS in sandbox

Workaround at src/libstore/meson.build#L429-L434 by @Ericson2314 from
NixOS#11302 erroneously used `macos` instead
of `darwin` to distinguish macOS, while meson docs list only `darwin`:
https://mesonbuild.com/Reference-tables.html#operating-system-names.

Original thread: NixOS#2503 (comment)

src/libstore/meson.build

@@ -427,7 +427,7 @@ extra_pkg_config_variables = {
 }
 
 # Working around https://github.com/mesonbuild/meson/issues/13584
-if host_machine.system() != 'macos'
+if host_machine.system() != 'darwin'
   extra_pkg_config_variables += {
     'localstatedir' : get_option('localstatedir'),
   }