ci: fix packaging tests
Start with branch head and then merge main rather than vice versa.
Trying to merge the branch head was resulting in the commit not being
found.
Additionally, the check for the series file needed to be negated.
Additional Context
Test Steps
Merge type
[x] Squash merge using "Proposed Commit Message"
[ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#)
Proposed Commit Message
Additional Context
Test Steps
Merge type