Previously, this was using outdated and 'insecure' syntax for setting an
environment variable. More info here:
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
This could have been solved by using `echo STUFF >> "$GITHUB_ENV"` but I
think using a step output is even better, mainly because it's more clear
what kind of changes `MADE_CHANGES` is talking about.
I also reordered the commands so that if writing the step output fails,
the step will fail before pushing the changes. That means we'll no
longer end up with half-released changes (released on GitHub but not
NPM).
