Tweak publish action to only build and push

[darronschall]

Previously the action was running bundle exec rake release, which was doing too much, and broke after #41 since the release task is meant to be run on a local development machine.

The GitHub publish_gem action automatically runs when a release tag is pushed. The only thing the action should do is build the gem and publish to RubyGems.

[danielmorrison]

Does this still use the trusted publisher stuff? This is all getting so confusing.

[darronschall]

I don't know the answer to that; I'm not entirely sure how GitHub authenticates to RubyGems as a trusted publisher.

Looking at the rubygem release action, there's an extra rubygems/configure-rubygems-credentials@v1.0.0 in there. That's probably required, I'll add that in.

[darronschall]

In order to get the action to run, I had to do a little bit of cleanup:

Delete the tag that the publish_gem action failed on:

% git tag -d v1.2.0 && git push --delete origin v1.2.0
Deleted tag 'v1.2.0' (was 59273e6)
To github.com:collectiveidea/protoc-gen-twirp_ruby.git
 - [deleted]         v1.2.0

Re-tag the latest after this PR was merged in:

% bundle exec rake release:source_control_push
Tagged v1.2.0.
Pushed git commits and release tag.

The push of the new 1.2.0 tag triggered the publish action to re-run for 1.2.0, which ran successfully and released 1.2.0 to RubyGems.