We should have been testing Dawn nightlies all along

[alexreinking]

As I discovered in halide/Halide#8714, our WebGPU support has bit-rotted pretty hard. We should have been testing with Dawn nightlies (or at least the newest chromium/NNNN branch) all along. The builds should have been managed by the build bots rather than by hand.

Picture unrelated.

[alexreinking]

The same is true for our acquisition of emsdk and node dependencies. All this should be managed by builders and none of it by hand.