nixos/coturn: refactor secret injection

The original implementation had a few issues:

* The secret was briefly leaked since it is part of the cmdline for
  `sed(1)` and on Linux `cmdline` is world-readable.
* If the secret would contain either a `,` or a `"` it would mess with
  the `sed(1)` expression itself unless you apply messy escape hacks.

To circumvent all of that, I decided to use `replace-secret` which
allows you to replace a string inside a file (in this case
`#static-auth-secret#`) with the contents of a file, i.e.
`cfg.static-auth-secret-file` without any of these issues.

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