There's a merged pull request on the void-packages repository

add xpkgdiff to CI
https://github.com/void-linux/void-packages/pull/36759

Description:
- common/travis/xpkgdiff.sh: add script to compare pkgs to previous version
- .github/workflows/build.yaml: add xpkgdiff CI step

#### Testing the changes
- I tested the changes in this PR: **YES**
  See demo PR: classabbyamp/void-packages#3
![image](https://user-images.githubusercontent.com/5366828/167016847-b5f1a57d-3f97-4f1e-9665-8277296e361c.png)