vagrant: Unbreak replacing symlinks on macOS

On a local installation on macOS, "cp -a" creates a write-protected
directory, which can't be renamed.  Do a "chmod +w" to unbreak.

Fixes #77671.

pkgs/development/tools/vagrant/default.nix

@@ -33,6 +33,8 @@ let
       for gem in "$out"/lib/ruby/gems/*/gems/*; do
         cp -a "$gem/" "$gem.new"
         rm "$gem"
+        # needed on macOS, otherwise the mv yields permission denied 
+        chmod +w "$gem.new"
         mv "$gem.new" "$gem"
       done
     '';