[emscripten] Switch Emscripten CI job to autoscaling runners
Summary:
Similar to D10328, D10339, D10340, but for Emscripten.
NOTE: We're using emscripten/emsdk:latest Docker image here instead of rust:1.74.0 like we are for the others.
Test Plan: See if CI passes
Reviewers: ashoat, ginsu, tomek, rohan, kamil
Reviewed By: ashoat, kamil
Differential Revision: https://phab.comm.dev/D10341