fix(jbep3): correct servercfgdefault to server.cfg (#4942)

The jbep3server details-check CI job fails because servercfgdefault was
set to "server.sample.cfg", but the Game-Server-Configs repo only ships
jbep3/server.cfg. The download step (curl -f) 404s and the job exits 22.

jbep3 was the only server using "server.sample.cfg"; all other
Source-engine servers (and 69 servers overall) use "server.cfg", which is
the file that actually exists in the configs repo.

lgsm/config-default/config-lgsm/jbep3server/_default.cfg

@@ -173,7 +173,7 @@ executabledir="${serverfiles}"
 executable="./srcds_run.sh"
 servercfgdir="${systemdir}/cfg"
 servercfg="${selfname}.cfg"
-servercfgdefault="server.sample.cfg"
+servercfgdefault="server.cfg"
 servercfgfullpath="${servercfgdir}/${servercfg}"
 
 ## Backup Directory