Solution from https://stackoverflow.com/questions/50714638/install-extension-from-a-specific-repo-branch-on-github
Install vsce: Make sure you have Node.js installed. Then run:
npm install -g vsce
Check out the GitHub repo/branch you want.
Depending on the project, you may need to install its dependencies (npm install
or whatever package manager you use). Some can be packaged without dependencies.
Run the following in the root of the project (see the official docs for more detail about the process):
vsce package # Generates a .vsix file
code --install-extension my-extension-0.0.1.vsix