deb-tests-focal CI job flakes with connection failure

[eloquence]

Example failure: https://app.circleci.com/pipelines/github/freedomofpress/securedrop/1056/workflows/988b91e4-5748-497d-a37a-10b509c42775/jobs/45010

Example output:

TASK [build-ossec-deb-pkg : Download OSSEC signature.] *************************

fatal: [focal-sd-generic-ossec-agent]: FAILED! => {"changed": false, "dest": "/tmp/build/ossec-hids-3.6.0.tar.gz.asc", "elapsed": 10, "msg": "Connection failure: The read operation timed out", "url": "https://github.com/ossec/ossec-hids/releases/download/3.6.0/ossec-hids-3.6.0.tar.gz.asc"}

[conorsch]

Submitted a PR with an attempted quick-fix, just increasing the timeout on this and related tasks. Stepping back, it's not really necessary that we build the ossec deb packages from scratch each run. Since they change so seldom, we could just fetch them from the FPF apt server, which would decrease builds times by a few minutes, at a guess.