Github Workflows don't run when using secrets.GITHUB_TOKEN, request to support SSH Deploy Key

**Is your feature request related to a problem? Please describe.**
When the PR is created, workflows that normally would run in on: pull_request don't run. This is expected behavior when using {{ secrets.GITHUB_TOKEN }}. A way to get around this when making commits is with a deploy key with write access.

**Describe the solution you'd like**
Add another parameter for the SSH key, if present, use that to make the commit (instead of the API).

Given that release-please does multiple things, including things that need a token, it seems this could be a backwards compatible change.

**Describe alternatives you've considered**
Create a PAT, but this is a brittle solution, as it is tied to a user, instead of the repo itself.

**Additional context**
sibling issue: https://github.com/googleapis/release-please-action/issues/1179