ci: add release-please token to fix stalled releases (#155)

This adds the release-please token to the release workflow to fix an
issue that occurred in #154

Looks like release-please is able to create a release PR, but not able
to start workflows using the default `GITHUB_TOKEN`.

- googleapis/release-please-action#818
- https://github.com/marketplace/actions/release-please-action
- peter-evans/create-pull-request#48

.github/workflows/release.yml

@@ -18,3 +18,4 @@ jobs:
           # this is a built-in strategy in release-please, see "Action Inputs"
           # for more options
           release-type: simple
+          token: ${{ secrets.RELEASE_PLEASE_ACCESS_TOKEN }}