feat(core): read github mirror address from environment variable

[subframe7536]

Description

Add support to read GitHub mirror address from environment variable

Motivation and Context

Many apps will download from GitHub, and sometimes the speed is slow (especially in China Mainland), so just add support to use custom GitHub mirror

How Has This Been Tested?

I have already edit the file on my machine locally, and everything works well

Checklist:

• [x] I have read the Contributing Guide.
• [x] I have ensured that I am targeting the develop branch.
• [ ] I have updated the documentation accordingly.
• [ ] I have updated the tests accordingly.
• [ ] I have added an entry in the CHANGELOG.