feat: worker: improve logtail by always getting the first 5 lines of a log
The first 5 lines look like this:
```
0s autopkgtest [23:08:03]: starting date and time: 2024-04-07 23:08:03+0000
0s autopkgtest [23:08:03]: git checkout: 31124158 autopkgtest: take Paride's WIP change regarding wrong src pkg selection
0s autopkgtest [23:08:03]: host juju-7f2275-prod-proposed-migration-environment-3; command line: /home/ubuntu/autopkgtest/runner/autopkgtest --output-dir /tmp/autopkgtest-work.22jn7d5i/out --timeout-copy=6000 --setup-commands /home/ubuntu/autopkgtest-cloud/worker-config-production/setup-canonical.sh --apt-pocket=proposed --apt-upgrade libreoffice --timeout-short=300 --timeout-copy=20000 --timeout-test=20000 --timeout-build=20000 '--env=ADT_TEST_TRIGGERS=mysql-8.0/8.0.36-2ubuntu3 redland/1.0.17-3.1ubuntu3' -- ssh -s /home/ubuntu/autopkgtest/ssh-setup/nova -- --flavor autopkgtest-big --security-groups <email address hidden> --name adt-noble-amd64-libreoffice-20240407-230803-juju-7f2275-prod-proposed-migration-environment-3-510113db-c395-457e-93e4-190cd04588df --image adt/ubuntu-noble-amd64-server --keyname testbed-juju-7f2275-prod-proposed-migration-environment-3 --net-id=net_prod-proposed-migration -e TERM=linux -e ''"'"'http_proxy=http://squid.internal:3128'"'"'' -e ''"'"'https_proxy=http://squid.internal:3128'"'"'' -e ''"'"'no_proxy=127.0.0.1,127.0.1.1,login.ubuntu.com,localhost,localdomain,novalocal,internal,archive.ubuntu.com,ports.ubuntu.com,security.ubuntu.com,ddebs.ubuntu.com,changelogs.ubuntu.com,launchpadlibrarian.net,launchpadcontent.net,launchpad.net,10.24.0.0/24,keystone.ps5.canonical.com,objectstorage.prodstack5.canonical.com'"'"'' --mirror=http://ftpmaster.internal/ubuntu/
61s autopkgtest [23:09:04]: testbed dpkg architecture: amd64
62s autopkgtest [23:09:05]: testbed apt version: 2.7.14build2
```
They are thus very valuable in many different situation:
* a job seems stuck, any developer can grab the command line and start
to debug this locally without waiting for the job to unstuck.
* the starting date and time can help autopkgtest admin to detect jobs
behaving weirdly, like looping.
* displaying the autopkgtest version will help developers know exactly
what the infra is running, and help admins quickly ensure that all
workers are running the intended version, for example.
* also dumping the testbed architecture and apt version can help detect
issues with testbeds, particularly with foreign architecture testing.
This is because the vimrc content is not part of the config, thus we
cannot detect when it changes. It may not be ideal, but it's fairly
quick, and will do its job for now.
