Unable to precompile assets when updating to `3.0.0.beta`

[codealchemy]

Describe the bug

Attempting to run bundle exec rake assets:precompile raises an exception

Reproduction steps

Update gem in a project using :sprockets, attempt to precompile

Expected behavior

Assets to be compiled as before

Additional context

• rails version: 6.0.5
• rails_admin version: 3.0.0.beta
• rails_admin npm package version: N/A
• full stack trace (if there's an exception)

(reduced this down to the essential bit of the error as we have internal middlewares in addition to those from Rails)

FrozenError:
  can't modify frozen Array: [... middleware classes ]
/project_dir/vendor/bundle/ruby/2.7.0/gems/actionpack-6.0.5/lib/action_dispatch/middleware/stack.rb:126:in `push'
/project_dir/vendor/bundle/ruby/2.7.0/gems/actionpack-6.0.5/lib/action_dispatch/middleware/stack.rb:126:in `use'
/project_dir/vendor/bundle/ruby/2.7.0/gems/<internal gem>:in `block in <class:Engine>'
/project_dir/vendor/bundle/ruby/2.7.0/gems/railties-6.0.5/lib/rails/initializable.rb:32:in `instance_exec'
/project_dir/vendor/bundle/ruby/2.7.0/gems/railties-6.0.5/lib/rails/initializable.rb:32:in `run'
/project_dir/vendor/bundle/ruby/2.7.0/gems/railties-6.0.5/lib/rails/initializable.rb:61:in `block in run_initializers'
/project_dir/vendor/bundle/ruby/2.7.0/gems/railties-6.0.5/lib/rails/initializable.rb:60:in `run_initializers'
/project_dir/vendor/bundle/ruby/2.7.0/gems/railties-6.0.5/lib/rails/application.rb:363:in `initialize!'

This appears to be related to changes to initialization from e59a935

Workaround

Keep only the precompile initializer during precompilation

# application.rb

if ENV.fetch('PRECOMPILE', false)
  RailsAdmin::Engine.initializers.keep_if { |i| i.name == 'RailsAdmin precompile hook' }
end

[codealchemy]

I was able to narrow this down to the RailsAdmin apply configuration initializer - my issue looks to be similar to #3533 where the change from e59a935 causes issues with other engines that affect application middleware. Closing to consolidate under #3533