onnxruntime

Add CI and PR validation triggers to QNN Windows x64 Pipeline yaml (#17178)

### Description
Adds continuous integration and pull-requestion validation triggers
directly to the yaml file for the Windows x64 QNN CI Pipeline.


### Motivation and Context
There have been various unit tests failures that break the
QNN_Windows_Nuget pipeline, which builds QNN EP for Windows x64. This PR
ensures that QNN EP is built and tested on a Windows x64 image for every
pull request.