Make github action builds path aware

[ganeshgore]

I am not sure if we have limited minutes for github actions, but complete build is not required for all pull requests. Please make github actions execution aware of changed paths. Pushing PostPnR doesn't need to test OpenFPGA build and verilog generation

Reference https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions#onpushpull_requestpaths