Since we can now override settings through environment variables, it's handy to contain those changes in a separate file. This way, sysadmins can generate such a file without having to touch the systemd file.
Meanwhile the current flags are converted to `Environment` entries, which can be overridden in the `EnvironmentFile` if so desired.
If the file is not present, the directive [is ignored](https://www.freedesktop.org/software/systemd/man/systemd.exec.html#EnvironmentFile=)
