Update Emscripten CI

[anutosh491]

1. Clones emsdk and uses it rather than fetching emsdk from conda-forge (just like xeus-python, xeus-cpp etc)

2. I think CMAKE_FIND_ROOT_PATH_MODE_PACKAGE is being used wrongly.

Cmake docs say that there are only 3 values this variables accepts

i) ONLY
ii) NEVER
iii) BOTH

What we are using is something like CMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ON which doesn't make sense.

As the docs say CMAKE_FIND_ROOT_PATH_MODE_PACKAGE basically controls two variables
i) CMAKE_FIND_ROOT_PATH
ii) CMAKE_SYSROOT

As far as my understanding goes we just need to control the CMAKE_FIND_ROOT_PATH that cmake uses while cross-compiling.

When cross-compiling (for example, when using Emscripten), CMake normally uses the host system's root directory as the search root. CMAKE_FIND_ROOT_PATH allows you to override this behaviour and specify alternate directories that CMake should treat as the root for package discovery.

So in this case we don't even need to control both vars through CMAKE_FIND_ROOT_PATH_MODE_PACKAGE as CMAKE_SYSROOT is not really of use to us.

[anutosh491]

cc @martinRenou

[martinRenou]

Yes 👍🏽 This one was a nasty hack that I don't want to maintain 😄

[anutosh491]

thanks for the review.

Feel free to merge once you're happy ;)

[martinRenou]

Thanks!