Expand envvars in topology_prefix configuration option

Author: vkarak

reframe/frontend/autodetect.py

@@ -204,7 +204,7 @@ def _emit_custom_script(job, env, commands):
 def detect_topology():
     rt = runtime.runtime()
     detect_remote_systems = rt.get_option('general/0/remote_detect')
-    topo_prefix = rt.get_option('general/0/topology_prefix')
+    topo_prefix = osext.expandvars(rt.get_option('general/0/topology_prefix'))
     for part in rt.system.partitions:
         getlogger().debug(f'detecting topology info for {part.fullname}')
         found_procinfo = False