Retry on transient fetch errors

In order to bypass:

Cloning repository...
Cloning into '/builds/gitlab/omnibus-gitlab'...
error: RPC failed; curl 56 SSL read: error:00000000:lib(0):func(0):reason(0), errno 104
fatal: The remote end hung up unexpectedly
fatal: early EOF
fatal: index-pack failed

Documentation on the variable: https://docs.gitlab.com/ee/ci/yaml/#job-stages-attempts