refactor(nix): Use nixpkgs' vesktop now that it supports darwin

nix/overlays/vesktop.nix

@@ -0,0 +1,13 @@
+final: prev:
+
+{
+  # Replace the icon with a much more low profile icon
+  # REF: https://github.com/Vencord/Vesktop/pull/865
+  vesktop = prev.vesktop.overrideAttrs (old: {
+    preBuild =
+      old.preBuild
+      + ''
+        cp -f ${../../include/vesktop/icon.icns} build/icon.icns
+      '';
+  });
+}