We still use Emscripten 1.39.9 for official Mono builds so ideally we want to test
against an old Emscripten version to ensure we don't break compatibility.
But then google-closure-compiler-linux broke compatibility for us and is not properly
pinned, so we need to use a more recent version for now to fix CI.
Cf. https://github.com/emscripten-core/emsdk/issues/802
This was forgotten when renaming 3.2.4 to 3.3.
Once 3.3-stable is out and we branch off to 3.3, the BASE_BRANCH should
be bumped to 3.3 in that branch.
We often hit "Too Many Requests" errors when uploading the cache with
`actions/cache` because there's a limit of 10 GB every 5 minutes, and we can
easily go over it when we amend or merge several PRs in a short timespan.
This will make the CI artifacts less useful for debugging crashes but there's
no real way around this.
(cherry picked from commit caea551d41)
Removes AppVeyor and all Travis jobs but the iOS one, which hasn't been ported
to GitHub Actions yet (should be done soon).
Backports new style scripts from `master` branch too to do the same checks.
