Enhance assets:precompile on Heroku to add non fingerptinted filenames - ruby-on-rails

Question

In a Rails 4 app, I want precompiled assets to have copies of some assets to be available with a non fingerprinted filename. The app is deployed on Heroku.
I have the following rake task in place:
namespace :app do
task :nonfingerprint_assets => :environment do
manifests = Rails.application.config.assets[:precompile]
manifests = manifests.slice(1, manifests.length)
assets = Dir.glob(File.join(Rails.root, 'public/assets/**/*'))
regex = /(-{1}[a-z0-9]{32}*\.{1}){1}/
assets.each do |file|
next if File.directory?(file) || file !~ regex
source = file.split('/')
source.push(source.pop.gsub(regex, '.'))
non_digested = File.join(source)
if manifests.any? {|m|
if m.is_a? Regexp
m.match non_digested
else
m.ends_with?(non_digested)
end
}
puts "** copy source: #{file}"
puts "** copy destination: #{non_digested}"
FileUtils.cp(file, non_digested)
end
end
end
end
Rake::Task['assets:precompile'].enhance do
puts "Invoke nonfingerprint_assets"
Rake::Task['app:nonfingerprint_assets'].invoke
end
This rake task is working fine on my local machine even in production environment. It creates a file in public/assets/application.js which is the copy of the same file with a digest in its name.
When I deploy this code to heroku I get the following error message which I don't understand:
-----> Preparing app for Rails asset pipeline
Running: rake assets:precompile
[deprecated] I18n.enforce_available_locales will default to true in the future. If you really want to skip validation of your locale you can set I18n.enforce_available_
locales = false to avoid this message.
Invoke nonfingerprint_assets
** copy source: /tmp/build_48b86bff-83f9-4d21-8b21-712c1baef811/public/assets/application-51d1660b66655782ab137ee98c789fdd.js
** copy destination: /tmp/build_48b86bff-83f9-4d21-8b21-712c1baef811/public/assets/application.js
rake aborted!
No such file or directory - /tmp/build_48b86bff-83f9-4d21-8b21-712c1baef811/public/assets/application.js
/tmp/build_48b86bff-83f9-4d21-8b21-712c1baef811/lib/tasks/production.rake:26:in `block (3 levels) in <top (required)>'
/tmp/build_48b86bff-83f9-4d21-8b21-712c1baef811/lib/tasks/production.rake:9:in `each'
/tmp/build_48b86bff-83f9-4d21-8b21-712c1baef811/lib/tasks/production.rake:9:in `block (2 levels) in <top (required)>'
/tmp/build_48b86bff-83f9-4d21-8b21-712c1baef811/lib/tasks/production.rake:45:in `block in <top (required)>'
Tasks: TOP => app:nonfingerprint_assets
(See full trace by running task with --trace)
Please help me to understand what's going on.