maintainers/scripts/update-ruby-packages: force platform-independent bundler lock

Since Bundler 2+, the lock command generated platform-dependent
Gemfile.locks, which breaks when nix has to build gems from source,
because the gemset generated is tied to the platform which generated it.

maintainers/scripts/update-ruby-packages

···
 (
   cd pkgs/development/ruby-modules/with-packages
   rm -f gemset.nix Gemfile.lock
-  bundle lock
+  # Since bundler 2+, the lock command generates a platform-dependent
+  # Gemfile.lock, hence causing to bundix to generate a gemset tied to the
+  # platform from where it was executed.
+  BUNDLE_FORCE_RUBY_PLATFORM=1 bundle lock
   bundix
   mv gemset.nix ../../../top-level/ruby-packages.nix
   rm -f Gemfile.lock