LLVM rebuilds instead of using cache on aarch64 with NiX

[adam-3bian]

When using the Nix environment on aarch64 with Debian Bookworm, LLVM is being recompiled locally instead of retrieved from the binary cache. This may be expected behaviour, as the system is not based on Ubuntu.

[nbdd0121]

Can you check if your user is added as a trusted user? See https://lowrisc.github.io/sonata-software/doc/getting-started.html#setup-nix-cache