Assets precompile fails on EC2: can't find jQuery - ruby-on-rails

Question

I'm configuring a new production deployment setup with a skeleton rails app, which I just generated using rails new with Rails 3.2.8. I have a production setup on my dev machine with apache2 and Phusion Passenger, which all works fine. When I try to push that configuration to EC2, the assets precompile step fails because it can't find jQuery.
Gemfile
source 'https://rubygems.org'
gem 'rails', '3.2.8'
gem 'capistrano'
gem 'jquery-rails'
gem 'haml'
group :assets do
gem 'less-rails'
gem 'uglifier', '>= 1.0.3'
gem 'therubyracer'
gem 'execjs'
end
group :deploy do
gem 'passenger'
end
group :production do
gem 'bigdecimal'
end
Relevant bit of deploy.rb
after "deploy:update_code" do
run "cd #{release_path}; rake assets:precompile RAILS_ENV=production "
end
Output
triggering after callbacks for `deploy:update_code'
* executing "cd /home/ec2-user/BWW/pages/bw-widget/releases/20121017095008; rake assets:precompile RAILS_ENV=production"
servers: ["bww-test"]
[bww-test] executing command
** [out :: bww-test] /usr/bin/ruby1.9 /usr/local/bin/rake assets:precompile:all RAILS_ENV=production RAILS_GROUPS=assets
** [out :: bww-test]
** [out :: bww-test] rake aborted!
** [out :: bww-test]
** [out :: bww-test] couldn't find file 'jquery'
** [out :: bww-test] (in /home/ec2-user/BWW/pages/bw-widget/releases/20121017095008/app/assets/javascripts/application.js:13)
** [out :: bww-test]
** [out :: bww-test]
** [out :: bww-test] Tasks: TOP => assets:precompile:primary
** [out :: bww-test]
** [out :: bww-test] (See full trace by running task with --trace)
** [out :: bww-test]
** [out :: bww-test] rake aborted!
** [out :: bww-test] Command failed with status (1): [/usr/bin/ruby1.9 /usr/local/bin/rake asset...]
** [out :: bww-test]
** [out :: bww-test] Tasks: TOP => assets:precompile
** [out :: bww-test] (See full trace by running task with --trace)
command finished in 1819ms
*** [deploy:update_code] rolling back
* executing "rm -rf /home/ec2-user/BWW/pages/bw-widget/releases/20121017095008; true"
servers: ["bww-test"]
[bww-test] executing command
command finished in 168ms
failed: "sh -c 'cd /home/ec2-user/BWW/pages/bw-widget/releases/20121017095008; rake assets:precompile RAILS_ENV=production'" on bww-test
If I look on the server, the bundle including jQuery has been installed by Capistrano:
[ec2-user#ip-10-226-122-140 bw-widget]$ find . -name jquery\*
./shared/bundle/ruby/1.9.1/specifications/jquery-rails-2.1.3.gemspec
./shared/bundle/ruby/1.9.1/cache/jquery-rails-2.1.3.gem
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3/jquery-rails.gemspec
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3/lib/generators/jquery
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3/lib/jquery
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3/lib/jquery-rails.rb
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3/vendor/assets/javascripts/jquery_ujs.js
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3/vendor/assets/javascripts/jquery-ui.js
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3/vendor/assets/javascripts/jquery.min.js
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3/vendor/assets/javascripts/jquery.js
./shared/bundle/ruby/1.9.1/gems/jquery-rails-2.1.3/vendor/assets/javascripts/jquery-ui.min.js
./shared/bundle/ruby/1.9.1/gems/rdoc-3.12/lib/rdoc/generator/template/darkfish/js/jquery.js
I'm slightly puzzled that the gems are in ruby/1.9.1 when I'm actually using 1.9.3p0 (the yum install version, not the rvm version), but I don't know if that is significant.
Update
Following a comment from Said, I changed the after update task to:
after "deploy:update_code" do
run "cd #{release_path}; bundle exec rake assets:precompile RAILS_ENV=production "
end
However, none of the documents I've read have suggested that bundle exec should be necessary, so I'm still concerned that while I have a work-around, something still isn't quite configured right.