Update Win_GPU_CI trigger (#13290)
### Description
supplement of #13248
Add PR trigger
https://learn.microsoft.com/en-us/azure/devops/pipelines/repos/github?view=azure-devops&tabs=yaml#pr-triggers
fix: master -> main
Testted with #13289 #13292
NB:
the real pipeline is always triggered if the workflow yaml changed even
it's added in the path filter.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Make sure the real pipeline not run in the backend.
