- For running on AOT tests on helix, we need to send emsdk as a payload
- On the CI machines, emscripten is available in `/usr/local/emscripten/`
- but the helix tasks try to write a `.payload` file in that dir, which fails
  because of permissions

- So, we copy emsdk to local `src/mono/wasm/emsdk`
- But we were doing it as part of *every* test build!
    - this meant unncessarily copying, and races causing errors like:

```
/__w/1/s/eng/testing/tests.wasm.targets(138,5):
    error MSB3026: Could not copy "/usr/local/emscripten/emsdk/node/14.15.5_64bit/bin/node" to "/__w/1/s/src/mono/wasm/emsdk/node/14.15.5_64bit/bin/node".

    Beginning retry 1 in 1000ms. The process cannot access the file '/__w/1/s/src/mono/wasm/emsdk/node/14.15.5_64bit/bin/node' because it is being used by another process.  [/__w/1/s/src/libraries/System.Collections.Specialized/tests/System.Collections.Specialized.Tests.csproj]
```

- Instead, do that once when preparing work items for helix

Fixes dotnet#52254 .