Add job for building PathKit to WASM using Docker

This creates a new named cache docker for this and future
docker-based jobs to use, to avoid permission snafus with
the normal "work" named cache.

Remove old WASM build, which was using the janky CIPD
emsdk asset and wasn't really exercising what we needed.

Bug: skia:8216
Change-Id: I993bba38b4978ca5eebb97e5b5b21729d55a072d
Reviewed-on: https://skia-review.googlesource.com/145140
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
19 files changed