checkout master tracking it CI upgrade test

[juagargi]

CI runs the upgrade test by building master. To do so, it runs

git checkout master
git pull --ff-only  # may be stale

But it could happen that master was already checked out in .git and recover by the cache, and thus the branch will be local only (or whatever the cache had) and git pull will fail.

Instead run

git checkout --force -B master --track origin/master

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This change is