move s6-rc db creation to a module

modules/s6-rc.nix

@@ -0,0 +1,17 @@
+{ config, pkgs, ... }:
+let
+  s6-rc-db = pkgs.s6-rc-database.override {
+    services = builtins.attrValues config.services;
+  };
+  inherit (pkgs.pseudofile) dir symlink;
+in {
+  config = {
+    environment = dir {
+      etc = dir {
+        s6-rc = dir {
+          compiled = symlink "${s6-rc-db}/compiled";
+        };
+      };
+    };
+  };
+}