Resolve "funkwhale-worker.service error option --concurrency"

Closes #1160 (closed)

While that issue was automatically closed, it was still very much present :)
As stated by @iomintz in the initial thread, systemd does not handle the ${VAR-DEFAULT} substitution (even newer versions). One correct way to do this is to set a default value with an Environment="VAR=DEFAULT" clause, which will be overridden if the variable is present in the environment file.

I tested this on my instance with and without defining the variable.

Thanks everyone for the good work !