tell git not to use terminal prompts

[dimbleby]

Fixes #3222

See https://git-scm.com/docs/git#Documentation/git.txt-codeGITTERMINALPROMPTcode

This environment variable is available since git 2.3 (2015) but should be harmless even in the unlikely event that someone has an older version than that.

[dimbleby]

probably also fixes #2825

[dimbleby]

Behaviour before on poetry lock is to sit there forever at "Resolving dependencies", after is:

$ poetry lock
Updating dependencies
Resolving dependencies... (0.6s)

Failed to clone https://github.com/rienafairefr/flask_pika.git, check your git configuration and permissions for this repository.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.