Allow JXL in non-nightly builds

By default, enabling JPEG XL is only possible in nightly builds. This allows enabling it in regular builds as well (and enables the build option; hopefully I got all the places to add it?)

It could also be considered to enable image.jxl.enabled in the settings by default as well, so it would "just work". There are no obvious downsides to that approach that I know of.

https://jpegxl.io/ and https://jpegxl.info/ (and maybe https://cloudinary.com/blog/the-case-for-jpeg-xl) for more info on what it's about, if needed.