Recent Emscripten versions no longer work on Netlify builders

Looks like we broke compatibility with Python 2.7 (as provided by Netlify builders):

```
5:41:27 PM: emcc -g4 --source-map-base https://emscripten-dbg-stories.netlify.app/ -o dist/crbug-837572.html ./crbug-837572.c
5:41:27 PM: Traceback (most recent call last):
5:41:27 PM: File "/tmp/tmp.YlmbiKiP0x/emsdk/upstream/emscripten/emcc.py", line 40, in
5:41:27 PM: import emscripten
5:41:27 PM: File "/tmp/tmp.YlmbiKiP0x/emsdk/upstream/emscripten/emscripten.py", line 463
5:41:27 PM: args.append(f'--separate-data-segments={memfile}')
5:41:27 PM: ^
5:41:27 PM: SyntaxError: invalid syntax
```