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
• [3.14] gh-145177: Support multiple Emscripten versions for Emscripten buildbot (GH-145180) #145582
• gh-145177: Put node version into emscripten/config.toml as well. #146156
• gh-145177: Add make-dependencies command to emscripten build script #146158
• [3.14] gh-145177: Put node version into emscripten/config.toml. (GH-146156) #146159
• gh-145177: Add emscripten run --test, uses test args from config.toml #146160
• [3.14] gh-145177: Add make-dependencies command to emscripten build script (GH-146158) #146184
• [3.14] gh-145177: Add emscripten run --test, uses test args from config.toml (GH-146160) #146186
• gh-145177: Fix Emscripten help text #150874
• [3.15] gh-145177: Fix Emscripten help text (GH-150874) #150894
• [3.14] gh-145177: Fix Emscripten help text (GH-150874) #150895
• gh-145177: Bump emscripten version to 5.0.7 #150896
• gh-145177: Bump emscripten version to 4.0.19 #150926
• [3.15] gh-145177: Bump emscripten version to 4.0.19 (GH-150926) #150939
• gh-145177: Bump emscripten to 6.0.0 #151421