Minor build script & GitHub Actions improvements

[NeRdTheNed]

• Uses Gradle toolchains to find a Java 8 JDK when building
• Improves build reproducibility by configuring Gradle to produce reproducible archives & not using cached files when building release artifacts
• Changes the release action to upload all files in the build/libs folder, removing the need for a hardcoded name
• Minor formatting changes

[Huskydog9988]

maybe target rewrite so we don't have to backport?