fix: Add `--cores` to gh actions nextstrain build

[corneliusroemer]

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:

• https://github.com/nextstrain/cli/issues/272

• https://bedfordlab.slack.com/archives/C01LCTT7JNN/p1681324279363569?thread_ts=1681293140.124269&cid=C01LCTT7JNN

• [ ] Test run: https://github.com/nextstrain/forecasts-ncov/actions/runs/4682609939/jobs/8296675794