Make nextstrain.org PR from Auspice PR merge ref
Make nextstrain.org PR from Auspice PR merge ref

Previously, Auspice was installed from the source branch HEAD directly.
This can fail to catch issues if there are notable changes in the PR
target branch unincorporated in the source branch. Creating the
nextstrain.org PR from the auto-merged ref should surface any
merge-related issues sooner than later.

nextstrain/auspice,nextstrain/auspice,nextstrain/auspice