Add workaround for RuboCop 0.35.0 Etc.getlogin bug

If a builder executes the `rubocop` command with the latest version the
job fails with a `no implicit conversion of nil into String` error. This
is a known issue with this version and has a fix pending:
rubocop/rubocop#2407
rubocop/rubocop#2408

The easiest delivery-truck workaround is to ensure $USER is set in the
subprocess that executes the linting.

metadata.rb

@@ -4,7 +4,7 @@
 license          'Apache 2.0'
 description      'Delivery build_cookbook for your cookbooks!'
 
-version          '1.100.7'
+version          '1.100.8'
 
 source_url       'https://github.com/chef-cookbooks/delivery-truck'
 issues_url       'https://github.com/chef-cookbooks/delivery-truck/issues'

recipes/lint.rb

@@ -24,6 +24,10 @@
   # Run Rubocop against any cookbooks that were modified.
   execute "lint_rubocop_#{cookbook.name}" do
     command "rubocop #{cookbook.path}"
+    environment(
+      # workaround for https://github.com/bbatsov/rubocop/issues/2407
+      'USER' => (ENV['USER'] || 'dbuild')
+    )
     only_if { File.exist?(File.join(cookbook.path, '.rubocop.yml')) }
   end
 end