vagrant: Resolve crash by replacing gem symlinks with directories

This workaround was discovered by painstakingly resolving the
differences between a working installation and a non-working
installation until a minimal change was found.  I have no idea why it
works.

Fixes #76629.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

pkgs/development/tools/vagrant/default.nix

@@ -25,6 +25,17 @@ let
         inherit version;
       };
     } // lib.optionalAttrs withLibvirt (import ./gemset_libvirt.nix));
+
+    # This replaces the gem symlinks with directories, resolving this
+    # error when running vagrant (I have no idea why):
+    # /nix/store/p4hrycs0zaa9x0gsqylbk577ppnryixr-vagrant-2.2.6/lib/ruby/gems/2.6.0/gems/i18n-1.1.1/lib/i18n/config.rb:6:in `<module:I18n>': uninitialized constant I18n::Config (NameError)
+    postBuild = ''
+      for gem in "$out"/lib/ruby/gems/*/gems/*; do
+        cp -a "$gem/" "$gem.new"
+        rm "$gem"
+        mv "$gem.new" "$gem"
+      done
+    '';
   };
 
 in buildRubyGem rec {