Update jamulus-headless.service

[rdica]

This should be sufficient but feel free to improve if I've missed something.

Short description of changes

The program was being executed indirectly via a call from a bash shell. This shell was also intercepting SIGUSR2 signals used to start/stop recording from the command line per #1515 (comment)
This change updates the systemd service file to execute the program directly instead of from inside a shell.

CHANGELOG: Server: Fix systemd ExecStart command line to support start/stop recording signal

Context: Fixes an issue?

Does this change need documentation? What needs to be documented and how?

Status of this Pull Request

What is missing until this pull request can be merged?

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[ann0see]

I will test this hopefully soon on my server.

[ann0see]

Thanks.
In the meantime, please think of a changelog entry and edit your PR description with it.

[rdica]

Thanks. In the meantime, please think of a changelog entry and edit your PR description with it.

Done. Let me know if it's missing anything, or needs rewording, thanks.

[ann0see]

Ok. Sorry for the delay. But it works fine - as expected.