Get rid of fixed number cmake --build -j in sh and docs

### 📚 The doc issue

Right now we have a few places where the number of `cmake --build -j` is fixed.
https://github.com/search?q=repo%3Apytorch%2Fexecutorch+%22+-j%22&type=code

Ideally let's use 
mac: `$(sysctl -n hw.ncpu) - 1`
linux: `$(nproc) - 1`

Update these docs and scripts.

### Suggest a potential alternative/fix

_No response_

cc @larryliu0820 @jathu