Update cacher_dir location
Update cacher_dir location

While Replatforming search-api and disabling root containers from
running in EKS - we detected an error:

Errno::EACCES: Permission denied @ dir_s_mkdir - tmp
/usr/local/lib/ruby/2.7.0/fileutils.rb:250:in mkdir'

To resolve this issue i have updated to use /tmp rather then tmp.

https://trello.com/c/Jef1t4xU/903-fix-app-permission-errors-due-to-nonroot

alphagov/rummager,alphagov/rummager