Adds a comment to the systemd service noting the location of the java path as a runtime environmental variable

[Dannyzen]

Resolves #1151

Because the systemd service config in the install script does not contain the PATH of the java executable, the service fails to start. Opting to add this as a comment as paths pointing to a JRE install aren't always in /usr/bin/java

[mcm001]

Under what circumstances will /usr/bin/java not be correct? It seems to be on the images we currently build for pi and opi

[Gold856]

Closing as stale