Ensure test workflow runs against merge commit, not HEAD of feature branch

[patcon]

Re-ticketed from: #33 (comment)

It seems that the test workflow is being run against the feature branch rather than against the merge commit of feature+mainline.

[patcon]

Seems a little trickier than I expected, but I think it's fixable.

Will be helpful:

• our file: https://github.com/g0v-network/domains/blob/main/.github/workflows/domain-plan.yml
• origin of tactic: add_pr_comment adds new comment for each run solvaholic/octodns-sync#41 (comment)

[patcon]

Another related issue, which I resolved by rebasing #33 against main after the merge of the other PR (that added the new files):
https://github.com/g0v-network/domains/runs/5267184572?check_suite_focus=true