Don't try to generate cultivator token on PR runs

[bleggett]

Now that the cargo publish/release-plz flow is in place, we don't need to (and cannot) "publish" on PR runs.

This fixes spurious CI publish failures on PRs like:
#56

[found-it]

Good catch @bleggett, the idea here was that we run the workflow and build the styrolite artifacts then upload them to the workflow run in case anyone wants to download them. Rather than not running on PR, we should just use an if conditional to only generate a token and publish (last two jobs) on release events.

[bleggett]

Good catch @bleggett, the idea here was that we run the workflow and build the styrolite artifacts then upload them to the workflow run in case anyone wants to download them. Rather than not running on PR, we should just use an if conditional to only generate a token and publish (last two jobs) on release events.

Yep, that's better. I missed the artifact upload would still happen even on PRs.