Provide some lever for controlling installer error logging in CI environments

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Feature Request

Provide some kind of environment variables to control the Poetry installer's logging. The installer script installs to a poetry-installer-error*.log file, but in CI environments this can be painful to debug. There should be some kind of lever to pull to get poetry to output the installation errors directly to stderr.

Many packages automatically detect the CI variable (standard among the CI providers from what I can tell) to automatically change configuration parameters.

Start by locating the installer script and the code that writes poetry-installer-error*.log. Review how installer failures are currently reported, then check the project's existing environment-variable handling. Done means CI users have a documented way to route installer errors to stderr instead of relying only on the log file, with coverage for the chosen behavior.