fix: Add --cores to gh actions nextstrain build

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/run-models.yaml

@@ -71,4 +71,5 @@ jobs:
             envdir env.d snakemake \
               --configfile config/config.yaml optional.yaml \
               --printshellcmds \
+              --cores 8 \
               "${override_config[@]}"