gh-actions: emscripten switch to "tip of tree"

[mr-c]

This reverts commit 07eb419093e952fd48d8486a3b5a8e366ba5b055 now that https://github.com/emscripten-core/emscripten/issues/20250 is fixed.

Blocked by

• [x] https://github.com/llvm/llvm-project/issues/69820
• [x] https://github.com/emscripten-core/emscripten/issues/20505