Fix usage of user in systemd services

- All NixOS module configurations allowed the configuration of user,
  but never actually used that option. Fix this to allow the user to
  configure the name of the user used when running the systemd
  service.

Author: sevanspowell

nix/nixos/metadata-server.nix

@@ -105,6 +105,7 @@ in {
       serviceConfig = {
         ExecStart = config.services.metadata-server.script;
         DynamicUser = true;
+        User = config.services.metadata-server.user;
         RuntimeDirectory = "metadata-server";
         StateDirectory = "metadata-server";
       };

nix/nixos/metadata-sync.nix

@@ -111,6 +111,7 @@ in {
       serviceConfig = {
         ExecStart = config.services.metadata-sync.script;
         DynamicUser = true;
+        User = config.services.metadata-sync.user;
         RuntimeDirectory = "metadata-sync";
         StateDirectory = "metadata-sync";
       };

nix/nixos/metadata-webhook.nix

@@ -111,6 +111,7 @@ in {
       serviceConfig = {
         ExecStart = config.services.metadata-webhook.script;
         DynamicUser = true;
+        User = config.services.metadata-webhook.user;
         RuntimeDirectory = "metadata-webhook";
         StateDirectory = "metadata-webhook";
         StandardOutput = "journal";