[tor-bugs] #23755 [Core Tor/Tor]: conditionalize mirroring script in .gitlab-ci.yml

[Tor Bug Tracker & Wiki]

#23755: conditionalize mirroring script in .gitlab-ci.yml
------------------------------+-------------------------------------------
     Reporter:  catalyst      |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-ci continuous-integration
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 A few of us have experimented with using the `.gitlab-ci.yml` to run
 continuous integration jobs on gitlab.com and oniongit.eu.  Right now
 they're failing in the "update" job because `$DEPLOY_KEY` isn't
 configured.  At the very least we should make the job cleanly exit if the
 key is not configured.

 Typical output from the "update" job:

 {{{
 $ eval $(ssh-agent -s)
 Agent pid 4533
 $ ssh-add <(echo "$DEPLOY_KEY")
 Enter passphrase for /dev/fd/63: ERROR: Job failed: exit code 1
 }}}

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23755>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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