`batou_ext.cron.SystemdTimer`: values of `additional_service_config` are not quoted

[Ma27]

Given the following configuration:

self += batou_ext.cron.SystemdTimer(
  # ...
  additional_service_config = {
    "WorkingDirectory": "/var/lib/foo"
  }
)

the Nix configuration will look like this:

{
  # ...
  WorkingDirectory = /var/lib/foo;
}

This is dangerous because in some cases the entire directory would be copied into the Nix store. This is not only problematic because the store is world-readable, this can also slow down evaluation significantly.

In other words, I'd expect to get WorkingDirectory = "/var/lib/foo"; given the config above.