dev: Set CONDA_SUBDIR during build, unless it's already set

[tsibley]

This helps macOS users on arm64 chips use and produce osx-64 Conda packages instead of osx-arm64 packages (which we don't yet support).

Resolves https://github.com/nextstrain/conda-base/issues/10.

Testing

• [x] Overriding CONDA_SUBDIR manually works
• [ ] Checks pass