CI: Pin Emscripten version for reproducible builds

Emscripten is a fast-moving target which gets tons of improvements all the time,
but it's not rare that some regressions affect us and make our CI builds fail.
(See e.g. #33728, #35237, #39168, #40563, and #40914.)

Let's pin to a stable version to avoid having external factors impact our CI,
and update this version manually regularly in a PR to ensure that the new
version works well for us.