disable servant build flag for i386ancient

Its library stack is too old, and while lts-12.14 does include an old
version of servant, some libraries like http-client have been bumped up
from the lts to support eg http-client-restricted. So a newer servant
would be needed, which would lead to many more upgrades.

There might be a dependency set that works, but I have not been able to
find it so far. stack solver also failed to find one.

standalone/linux/stack-i386ancient.yaml

@@ -10,6 +10,7 @@ flags:
     debuglocks: false
     benchmark: false
     crypton: false
+    servant: false
 packages:
 - '.'
 extra-deps: