Add externals directory caching to GitHub workflow for faster builds#1225

jonathanpeppers merged 2 commits intomaindotnet/android-libraries:mainfrom

copilot/fix-75281ea0-d5f3-488e-b8e2-4cff1c7f2d4edotnet/android-libraries:copilot/fix-75281ea0-d5f3-488e-b8e2-4cff1c7f2d4eCopy head branch name to clipboard