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.
The base branch is hardcoded as an env variable as I couldn't find a simple
way to just get either `3.2` or `master`. But it's easy to change when we
branch off from `master` to a new stable branch, which doesn't happen often.
(There's `{{github.base_ref}}` but it's probably more verbose like
`ref/heads/master`, and only valid for PRs.)
