buildworker: start: fix worker startup failure after update

[ynezz]

buildworker: start: fix worker startup failure after update

Workers are currently refusing to work:

 Unhandled Error
 Traceback (most recent call last):
   File "/opt/venv/lib/python3.11/site-packages/twisted/application/app.py", line 673, in run
     runApp(config)
   File "/opt/venv/lib/python3.11/site-packages/twisted/scripts/twistd.py", line 29, in runApp
     runner.run()
   File "/opt/venv/lib/python3.11/site-packages/twisted/application/app.py", line 370, in run
     self.application = self.createOrGetApplication()
   File "/opt/venv/lib/python3.11/site-packages/twisted/application/app.py", line 437, in createOrGetApplication
     application = getApplication(self.config, passphrase)
 --- <exception caught here> ---
   File "/opt/venv/lib/python3.11/site-packages/twisted/application/app.py", line 446, in getApplication
     application = service.loadApplication(filename, style, passphrase)
   File "/opt/venv/lib/python3.11/site-packages/twisted/application/service.py", line 404, in loadApplication
     application = sob.loadValueFromFile(filename, "application")
   File "/opt/venv/lib/python3.11/site-packages/twisted/persisted/sob.py", line 174, in loadValueFromFile
     codeObj = compile(data, filename, "exec")
 builtins.SyntaxError: keyword argument repeated: connection_string (buildbot.tac, line 49)

as the buildbot.tac template changed in commit 4ba1dcb66155 ("worker:
Add option --connection-string for create-worker") and version v3.10.0.

So lets use this new --connection-string feature and get rid of the
sed-fu.

Fixes: efbddc9 ("Bump buildbot to v3.11.1 release")

ci: fix worker startup issue by adding missing env vars and cleanup unused vars

Workers are currently refusing to work:

  Unhandled Error
  Traceback (most recent call last):
    File "/opt/venv/lib/python3.11/site-packages/twisted/application/app.py", line 673, in run
      runApp(config)
    File "/opt/venv/lib/python3.11/site-packages/twisted/scripts/twistd.py", line 29, in runApp
      runner.run()
    File "/opt/venv/lib/python3.11/site-packages/twisted/application/app.py", line 370, in run
      self.application = self.createOrGetApplication()
    File "/opt/venv/lib/python3.11/site-packages/twisted/application/app.py", line 437, in createOrGetApplication
      application = getApplication(self.config, passphrase)
  --- <exception caught here> ---
    File "/opt/venv/lib/python3.11/site-packages/twisted/application/app.py", line 446, in getApplication
      application = service.loadApplication(filename, style, passphrase)
    File "/opt/venv/lib/python3.11/site-packages/twisted/application/service.py", line 404, in loadApplication
      application = sob.loadValueFromFile(filename, "application")
    File "/opt/venv/lib/python3.11/site-packages/twisted/persisted/sob.py", line 174, in loadValueFromFile
      codeObj = compile(data, filename, "exec")
  builtins.SyntaxError: keyword argument repeated: connection_string (buildbot.tac, line 49)

and we're not aware about it, so lets fix it by adding the same
environment variables we're actually using in production.

While at it, cleanup the unused container_verify_string variables.

Fixed (failing) CI test is visible here https://github.com/openwrt/buildbot/actions/runs/11060475202/job/30731095086#step:5:48