Modify TMPDIR env variable for each deploy to prevent filling HDD

Describe your PR
Each job gets its own TMPDIR within which the normal workflow applies. This gathers both the temporarily cloned repo and any temporary files/dirs Capistrano/Bundler/NPM create during deploys.

The HDD has been filling up on our deployment server every few weeks. And this is a more generic solution then creating a cron job to clean things up.

/cc @zendesk/samson @pswadi-zendesk 

### References
 - Jira link: https://zendesk.atlassian.net/browse/SAMSON-127

### Risks
 - None