Add PostgreSQL DB for performance test results

The database will hold the performance results of the execution of the
benchmarks. We follow the same setup on knights3 for now.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>

m/hut/configuration.nix

@@ -15,6 +15,7 @@
     ./public-inbox.nix
     ./gitea.nix
     ./msmtp.nix
+    ./postgresql.nix
     #./pxe.nix
   ];