fix: Add --cores all to rebuild-mpxv.yaml

This is required for the action to be compatible with
snakemake version >5.11.0 which started to require the
`--cores` option
For more context see:
- nextstrain/cli#272
-https://bedfordlab.slack.com/archives/C01LCTT7JNN/p1681324279363569?thread_ts=1681293140.124269&cid=C01LCTT7JNN

.github/workflows/rebuild-mpxv.yaml

@@ -35,6 +35,7 @@ jobs:
           . \
             envdir env.d snakemake notify_on_deploy \
               --configfiles config/config_mpxv.yaml config/nextstrain_automation.yaml \
+              --cores all \
               --printshellcmds
       env:
         AWS_DEFAULT_REGION: ${{ secrets.AWS_DEFAULT_REGION }}