Use preinstalled JDKs in GitHub Actions workflow
Use preinstalled JDKs in GitHub Actions workflow

This should both make runs faster and save data
(making builds a bit "greener")

Always use an explicit JAVA_HOME_8_X64 so this will
still work when ubuntu-latest switches to ubuntu-20.04.

Also fix the comment wrt the JDK version being used
in the Windows build.

tbroyer/gradle-errorprone-plugin,tbroyer/gradle-errorprone-plugin