This came up when running the main runtime build for os browser under our ryujit wasm test framework.

Doing a fresh runtime build with the clr+libs subset (but no mono subset) doesn't seem to pull down the emscripten sdk.

To reproduce, build a subset like:

.\build.cmd -c Debug -rc Release -os browser clr+libs

on a system with no emsdk in the path, and an error about locating emcmake (or similar) should be produced.

I'm opening this just in case this is something we want to change -- I imagine we want to be able to download the emscripten sdk for non-mono build subsets, if os=browser? cc: @pavelsavara