Improvements to the Emscripten buildbot

[hoodmane]

• Use the build script from its new Platform/emscripten/__main__.py location
• Use python3 Platform/emscripten run to avoid hard coding build output path
• Use new EMSDK_CACHE environment variable to allow build script to manage
  Emscripten installations
• Build libmpdec
• Cache libffi and libmpdec
• Allow build script to manage node installations

python/cpython#146156 has to land first

[vstinner]

python/cpython#146156 has to land first

It landed, but its 3.14 backport didn't land yet: python/cpython#146159

[hoodmane]

I guess we'll also want to merge and backport python/cpython#146158 and python/cpython#146160 first so we can move more configuration from here into the cpython repo.

[freakboy3742]

One typo; but otherwise this looks good to me. All the upstream PRs have landed, including 3.14 backports; and I've manually tested the commands on the buildbot.

[vstinner]

All the upstream PRs have landed, including 3.14 backports

Ok. I merged the PR (the new config will be deployed in a few minutes). Let's see how it goes :-)

• WASM Emscripten 3.x
• WASM Emscripten 3.14

[hoodmane]

Thanks @vstinner!

[hoodmane]

Well it is not quite right:
#688

[hoodmane]

Looks working now:
https://buildbot.python.org/all/#/builders/1809/builds/108

[mhsmith]

Since this PR was merged, the Emscripten buildbot has posted some comments on GitHub in which the "traceback logs" appears to be the entire stderr of the build and test process:

• gh-146228: Better fork support in cached FastPath cpython#146231 (comment)
• gh-144319: obtain SeLockMemoryPrivilege on Windows cpython#144928 (comment)

These were brought to my attention because I have a filter set up for the word "Android", which shows up in the log because the Android test has been (correctly) skipped. I haven't had this happen with other buildbots.

Separately, the logs are a bit mangled both in the GitHub comment and the Buildbot web interface because they include ANSI color codes. The other buildbots don't do this either.

[hoodmane]

Seems likely to be because the interpreter crashed? Presumably the bot looked for the pattern that it expects failing unittests to generate and didn't find it so instead it included all of stdout.