Suggest a branch environment variable for Github Actions

Currently, we already have a GITHUB_REF environment variable, but it’s a little not so easy to get the branch name easily.

I’d like to suggest adding a GITHUB_BRANCH to return the branch that triggered the current workflow, which is similar like the BUILD_SOURCEBRANCHNAME environment variable in Azure Pipeline

https://docs.microsoft.com/en-us/azure/devops/pipelines/build/variables?view=azure-devops&tabs=yaml#build-variables-devops-services