Use osx-arm64 packages on Apple silicon

[victorlin]

Description of proposed changes

Previously the osx-64 packages were used because osx-arm64 was not supported.

Related issue(s)

Can be used to test nextstrain/conda-base#80

Checklist

• Installation is successful:

  NEXTSTRAIN_CONDA_CHANNEL=nextstrain/label/pull-80
  NEXTSTRAIN_CONDA_BASE_PACKAGE="nextstrain-base ==20240712T150634Z" \
    nextstrain setup --force conda
  

• nextstrain shell --conda works

• Check that a CI workflow run is successful

• Merge feat: Make osx-arm64 version of conda-base conda-base#80

• Checks pass

  • Failure of CI / test-standalone (os=macos-14, target=aarch64-apple-darwin) is expected until previous task is completed