Nuget - Prefer APPVEYOR_REPO_BRANCH

[campersau]

Summary: After https://github.com/cefsharp/CefSharp/pull/4763 was merged I have seen in https://ci.appveyor.com/project/cefsharp/cefsharp/builds/49434312/artifacts that the branch name was HEAD instead of master. Looks like AppVeyor does checkout the commit rather than the branch: https://github.com/appveyor/ci/issues/1606

Changes:

• Added check for APPVEYOR_REPO_BRANCH and prefer it over the branch name we get from git.

How Has This Been Tested?
Manually

Screenshots (if appropriate):

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Updated documentation

Checklist:

• [X] Tested the code(if applicable)
• [ ] Commented my code
• [ ] Changed the documentation(if applicable)
• [ ] New files have a license disclaimer
• [ ] The formatting is consistent with the project (project supports .editorconfig)

[AppVeyorBot]

:white_check_mark: Build CefSharp 122.1.120-CI4968 completed (commit https://github.com/cefsharp/CefSharp/commit/2ba76bbff6 by @campersau)

[amaitland]

Thank you!