scansion tests: Allow specifying network backend via environment variable

To make it easier to test select and event without having to edit the
config file, e.g. in CI.

make integration-test PROSODY_NETWORK_BACKEND=event

spec/scansion/prosody.cfg.lua

@@ -13,7 +13,7 @@ end
 
 admins = { "admin@localhost" }
 
-network_backend = "epoll"
+network_backend = ENV_PROSODY_NETWORK_BACKEND or "epoll"
 network_settings = {
 }