[ci] Use newly official github action to install emsdk

[sbc100]

See https://github.com/emscripten-core/setup-emsdk

[kripken]

I wonder if there is value in testing the old way too? Just to have the explicit list of commands and not depend on that side repo with the github action. E.g. a typo in that side repo could lead to us testing latest stable and not tot.

Or, put another way: do we have tests that the github action is doing exactly what it should? This will test it works, but I mean that it installs tot?

[sbc100]

I wonder if there is value in testing the old way too? Just to have the explicit list of commands and not depend on that side repo with the github action. E.g. a typo in that side repo could lead to us testing latest stable and not tot.

Or, put another way: do we have tests that the github action is doing exactly what it should? This will test it works, but I mean that it installs tot?

We should have tests there yes.. but I don't think we need to block this change on that.. the main tested we do is still using circleci where we cannot/don't use these rules anyway.

[sbc100]

Also, because its the codesize tests specifically that we updating here.. we would know very quickly if it was not tot that was being installed because we could get inconsistent codesize results (i.e. these tests are very sensitive to tot being the installed SDK).

[kripken]

Nice, good point about the code size tests.