Which version of the book is affected?

Problem is with source files on main branch.

https://github.com/progit/progit2/tree/main/.github/workflows

Describe the bug:

Our GitHub Action is somehow still not creating .mobi releases.
It seems to be related to kindlegen, according to the Action log output.

Steps to reproduce:

Go to GitHub releases tab, see no .mobi files for download.
The Release on push to main action is giving a error message on the Build release assets step:

Converting to Mobi (kf8)...
Unable to run KindleGen. Either install the kindlegen gem or place `kindlegen` executable on PATH or set KINDLEGEN environment variable with path to it (RuntimeError)

Link to error message on a recent run: https://github.com/progit/progit2/runs/2942333250?check_suite_focus=true#step:5:13

Expected behavior:

.mobi files are building and releasing properly.

Screenshots:

Additional context:

Maybe try running the rake build script locally, to confirm it works.
I have a feeling that maybe the github action is barred from downloading from the web archive that we use to grab the kindlegen, but I can't confirm/deny this. 😄