Emscripten buildbot: Support multiple Emscripten versions

[hoodmane]

We want to allow different branches to use different Emscripten versions. The plan is as follows:

1. Add emscripten_version.txt file and add --emsdk-cache argument to emscripten/__main__.py. If --emsdk-cache is passed, the build script will require that the appropriate emsdk version is found in that directory.
2. Backport this to 3.14.x
3. Set up buildbot cache dir
4. Make a PR to buildmaster config to pass the --emsdk-cache argument

cc @freakboy3742

Linked PRs

• gh-145177: Support multiple Emscripten versions for Emscripten buildbot #145180