gnomeExtensions.system-monitor: fix system-monitor name collision

"system-monitor@paradoxxx.zero.gmail.com" is manually packaged,
"System_Monitor@bghome.gmail.com" is automatically packaged.
Both have the extensionPortalSlug "system-monitor", but the former
gets shadowed by the latter.

pkgs/desktops/gnome/extensions/extensionRenames.nix

@@ -13,6 +13,8 @@
   "lockkeys@vaina.lt" = "lock-keys";
   "lockkeys@fawtytoo" = "lock-keys-2";
 
+  "system-monitor@paradoxxx.zero.gmail.com" = "system-monitor"; # manually packaged
+  "System_Monitor@bghome.gmail.com" = "system-monitor-2";
 
 
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