fix: templating for /etc/sysconfig/ path

$prog is undefined at this point

src/main/resources/com/typesafe/sbt/packager/archetypes/java_server/systemloader/systemv/start-rpm-template

@@ -34,7 +34,7 @@
 
 # Source from sysconfig
 # This order means system config appends/overrides package config
-[ -e /etc/sysconfig/$prog ] && . /etc/sysconfig/$prog
+[ -e /etc/sysconfig/${{app_name}} ] && . /etc/sysconfig/${{app_name}}
 
 exec="${{chdir}}/bin/${{exec}}"
 prog="${{app_name}}"