Automatically PR to update poetry lockfile v2

[sneakers-the-rat]

Fixes: #2023

Sorry for the commit spray here - i would suggest doing a squash merge on this one. debugging stuff on CI is always so tedious.

on the maiden voyage of the lockfile updater, it became clear that something was amiss in the generated PR (there was nothing in it). serves me right for drawing from a gh action with like 5 usages.

now presenting, the new and improved version

• turns out you need to install the package before calling update, or else it just installs all the current versions without saying "updating" anywhere.
• you also need to specify bash as the shell explicitly, bc sh doesn't like stuff like "multiline strings"
• you also need to use this fancy multiline string syntax to store the variable
• and finally, you also seem to need to store it in $GITHUB_ENV rather than $GITHUB_OUTPUT despite what the docs say

i have confirmed that this works now on my own fork: sneakers-the-rat#4

thank you for bearing with me on this day where i was especially extra obnoxious

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.68%. Comparing base (aa75b4b) to head (33523b2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2028   +/-   ##
=======================================
  Coverage   80.68%   80.68%           
=======================================
  Files         104      104           
  Lines       11624    11624           
  Branches     2909     2909           
=======================================
  Hits         9379     9379           
  Misses       1701     1701           
  Partials      544      544           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.