Add Github Actions and enable Gradle build scans

[msgilligan]

• If CI, don’t upload Gradle buildscans in background
• Add 3 Github Actions workflows: gradle.yml, regtest.yml, graalvm.yml
• Refactor Omni Core download from .travis.yml to a BASH script named test-download-omnicore-ubuntu.sh
• Add README badges for 3 Github Actions workflows
• GithubCI RegTests upload test results as artifact (on failure, only)
• RegTest increase longerDelayAfterInvalidate (to 120 seconds!)
• Add Gradle caching *README: Add build note