Add option to skip emscripten version check

[ryanking13]

Adds an option to skip emscripten version check. A user can either set SKIP_EMSCRIPTEN_VERSION_CHECK env variable, or skip_emscripten_version_check in the toml file.

I think it is useful in following scenarios:

• When we update the Emscripten version in Pyodide but it is not released yet, so a user wants to build a package against newer emscripten version.
• When a user needs a newer version of Emscripten to make their package work. But the Emscripten version in Pyodide is not updated yet.

Of course, these scenarios will not work if two versions of Emscripten are incompatible. But I think it is useful to provide this option as not always

[agriyakhetarpal]

I think we should still check for (and allow) "-git" in the version string in addition to this change.

[ryanking13]

I think we should still check for (and allow) "-git" in the version string in addition to this change.

Right. Let's fix it in this PR too.

[agriyakhetarpal]

Thanks, @ryanking13!

[ryanking13]

Thanks for the review!