emsdk

Use "command -v" instead of "which" to detect python executable (#1419)

On a Linux distro that doesn't have the `which` program installed we're
getting the following error:

    $ ./emsdk install latest
    ./emsdk: line 39: exec: python: not found

It's failing to detect the installed `python3` and falls back to using
`python`, but this distro doesn't provide a python -> python3 symlink so
we fail.

Fix this by using `command -v` instead which is a POSIX standard.
The same change went into emscripten a couple years ago:
https://github.com/emscripten-core/emscripten/pull/15071