This is aarch64 (that is 64-bit ARM), and NixOS seems more 32-bit arm looking at their wiki https://nixos.wiki/wiki/NixOS_on_ARM On the other hand, they do 64-bit too, so maybe there would be some way to do for one, then bootstrap the other?

Other idea, distributions like Debian, seems to build OpenJDK as well, so maybe that would be a way to get the initial package (through an existing distro) https://buildd.debian.org/status/logs.php?pkg=openjdk-11 - see the armel/armhf/arm64 architectures (arm64 equals aarch64).