release: always use service user for git ops

[gulducat]

We are shoring up our user and bot accesses, and this seems to be our only usage of the github-actions bot.

These were the commits that have been flagged as needing adjustment:

workflows/release.yml
145:            git commit --message "Generate files for ${{ github.event.inputs.version }} release"
212:            git commit --message 'Prepare for next release'

We may later discover other of our repos that need a similar treatment, if they've just not been run during this information-gathering time, but at present this is the only known case. Our CE->ENT merge workflow in the ENT repo is already set up properly.