aurweb: Make SSH faster by avoiding slow Poetry (~2,5 sec faster)

"poetry run" is very slow[1] and adds +1 second to the startup time.
This is made even worse by the fact that aurweb-git-serve is called
twice by sshd[2].

[1] https://github.com/python-poetry/poetry/issues/3502
[2] https://security.stackexchange.com/questions/123795/authorizedkeyscommand-of-sshd-config-getting-called-twice/123801#123801