Race condition in primer cloned project caching

[jacobtylerwalls]

See logs showing a project (astroid) cached with the wrong SHA, because a commit landed in astroid between the steps:

Get commit string step:

'astroid' remote is at commit '5de983ac'.

Regenerate cache step:

Cloned 'astroid' at commit '680b40bfd2dc774af0a1047798cc98f06fce7a56'.

Post restore projects step (cache upload):

Cache saved with key: Linux-3.12-5de983ac_...