Extend 'default' task with RuboCop linting

This adds a new 'lint' rake task that runs as part of a default call
to 'rake'. Having linting run locally by default makes it easier to
catch build issues, and is a prerequisite for moving to GitHub Actions,
since we want to avoid listing lots of separate build steps.

Note that we first wipe the 'default' rake task, since RSpec has a habbit
of auto-populating it, which makes it hard to define it explicitly.

The 'lint' task runs RuboCop as a separate process, which is a technique
to avoid a LoadError when the gem is unavailable in production.

Rakefile

@@ -1,6 +1,7 @@
-# Add your own tasks in files placed in lib/tasks ending in .rake,
-# for example lib/tasks/capistrano.rake, and they will automatically be available to Rake.
-
 require File.expand_path("config/application", __dir__)
-
 Rails.application.load_tasks
+
+# RSpec shoves itself into the default task without asking, which confuses the ordering.
+# https://github.com/rspec/rspec-rails/blob/eb3377bca425f0d74b9f510dbb53b2a161080016/lib/rspec/rails/tasks/rspec.rake#L6
+Rake::Task[:default].clear unless Rails.env.production?
+task default: %i[spec lint]

lib/tasks/lint.rake

@@ -1,9 +1,4 @@
-unless Rails.env.production?
-  require "rubocop/rake_task"
-
-  RuboCop::RakeTask.new(:lint) do |t|
-    t.patterns = %w[app config Gemfile lib spec]
-    t.formatters = %w[clang]
-    t.options = %w[--parallel]
-  end
+desc "Lint Ruby"
+task lint: :environment do
+  sh "bundle exec rubocop"
 end