host(koumakan): postgres: use ident map for hydra

Changed files
+1
systems
koumakan
services
databases
+1
systems/koumakan/services/databases/postgresql.nix
···
authentication = lib.mkForce ''
# unix socket connection
+
local hydra all ident map=hydra-users
local all all peer
# local ipv4/6 tcp connection
host all all 127.0.0.1/32 scram-sha-256