nixos/home-assistant: Wait for {mysql,postgresql}.service

Database provisioning was shown to be racy since adding a recorder test
using PostgreSQL. There is no harm in waiting for these services,
because if they're not available they will be ignored.

nixos/modules/services/home-automation/home-assistant.nix

@@ -364,7 +364,13 @@ in {
 
     systemd.services.home-assistant = {
       description = "Home Assistant";
-      after = [ "network-online.target" ];
+      after = [
+        "network-online.target"
+
+        # prevent races with database creation
+        "mysql.service"
+        "postgresql.service"
+      ];
       preStart = optionalString (cfg.config != null) (if cfg.configWritable then ''
         cp --no-preserve=mode ${configFile} "${cfg.configDir}/configuration.yaml"
       '' else ''