Support --xbuildenv-path flag and PYODIDE_XBUILDENV_PATH environment variable with pyodide build

[agriyakhetarpal]

(Related to #72), and as stated in the issue title. I think we should allow this, as pyodide build at the moment downloads multiple cross-build environments (in whichever folder the command is run from, rather than using the existing one). If pyodide build can know where to look, we can specify that and not use excess bandwidth.

My question is, should we call it --xbuildenv-path/PYODIDE_XBUILDENV_PATH or --xbuildenv-dir/PYODIDE_XBUILDENV_DIR, i.e., path vs dir?

[ryanking13]

Sounds like a good idea to me.

My question is, should we call it --xbuildenv-path/PYODIDE_XBUILDENV_PATH or --xbuildenv-dir/PYODIDE_XBUILDENV_DIR, i.e., path vs dir?

Not a strong preference, but I would choose PYODIDE_XBUILDENV_PATH which is similar to PYTHONPATH and NODE_PATH.

[hoodmane]

Can we by default put it in xdg_cache_dir/ pyodide_xbuildenvs?

[agriyakhetarpal]

Yes, that would be fine. This option would be in conjunction with that – so that we can place it in a cache dir, but also allow placing it elsewhere if specified.

[ryanking13]

so that we can place it in a cache dir

+1. It would be useful to specify the xbuildenv path so people can cache it in CI.