[devops] make build on PR run automatically

[ver217]

Overview

Build on PR workflow only runs when a PR is labelled as "Run build and test". There are two main shortcomings:

• Author may forget to add the label.
• The order of label affects running times. E.g. I add two labels: "Run build and test" and "Bug". This will trigger twice CI. And I think this is an unexpected behavior.

We can solve these problems by triggering CI via path. The label is not needed anymore.

Goal

1. Make build on PR workflow run automatically.
2. Update related documentation.

[binmakeswell]

PR workflow only runs when a PR is labelled as "Run build and test".
This is for security reasons to avoid offensive PR according to FrankLee.

[ver217]

PR workflow only runs when a PR is labelled as "Run build and test". This is for security reasons to avoid offensive PR according to FrankLee.

We may not merge these PRs. Why it's harmful to unit test? By the way, author can add the label. How can you control this?