buildworker: start: fix worker startup failure after update

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 in version v3.10.0.

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

Fixes: efbddc90d8e6 ("Bump buildbot to v3.11.1 release")
Signed-off-by: Petr Štetiar <ynezz@true.cz>