Use UTF-8 file encoding

Append -Dfile.encoding=UTF-8 to org.gradle.jvmargs so the build reads and writes text the same way wherever it runs, rather than inheriting whatever default encoding the machine happens to have. Only an existing org.gradle.jvmargs is appended to: setting that property replaces Gradle's default daemon arguments rather than adding to them, so creating it to hold only an encoding would drop the default heap and metaspace settings. An encoding already set, UTF-8 or otherwise, is left as it is. See the Gradle best practice.