In order to better insulate our builds from changes to the platform defaults offered by GitHub Actions, we should specify the specific version of each OS and compiler rather than just using the defaults. This way we can choose when to upgrade and it will make it less likely for a build failure to start happening (e.g., JDK-8256978). The JDK project has already done this with JDK-8256393, JDK-8256354, and JDK-8256277.
I plan to do the same for JavaFX, with the exception of the MSVC compiler on Windows, which requires JDK-8255713 to be done, and is also more work (it increases the build setup time significantly, so we would need some sort of caching).
I propose to specify the following:
Linux: Ubuntu 20.04, gcc 10.2, ant 1.10.5
Mac: macOS 10.15, Xcode 11.3.1, ant 1.10.5
Windows: Windows Server 2019, ant 1.10.5