Phoenixd Version Installation - Phoenixd Already Installed

If phoenixd is already installed on that machine, the [install script](https://github.com/getAlby/hub/blob/master/scripts/linux-x86_64/phoenixd/install.sh) adds extra text to the existing systemd service file, causing errors.

To get albyhub to work with my phoenixd install, I had to fix that so that my prior installed phoenixd would start correctly.

Then had to update the albyhub start.sh file to point to the correct phoenix.conf file. Then it worked.

It could be helpful for the install script to ask if phoenixd is already installed. And if so, skip creating the systemd file and asking for the location of the phoenix.conf file to add to start.sh.

Start with scripts/linux-x86_64/phoenixd/install.sh and inspect the related start.sh handling for phoenix.conf. Reproduce the case where phoenixd is already installed, then verify the installer does not corrupt or recreate the existing systemd service and uses the existing configuration so Alby Hub starts correctly.

Written by the indexing model from the issue text.