julia.withPackages: add juliaCpuTarget option

[thomasjm]

Copying from the documentation I added for this feature:

• juliaCpuTarget: Allows you to set JULIA_CPU_TARGET when precompiling. Has no effect if precompile=false.

  You may want to use this if you're building a Julia depot that will end up in a Nix cache and used on machines with
  different CPUs.

  Why? Julia will detect the CPU microarchitecture of the build machine and include this information in the precompiled
  *.ji files. Starting in 1.10 Julia became more strict about checking the CPU target compatibility, so it may reject
  your precompiled files if they were compiled on a different machine.
  A good option to provide wide compatibility is to set this to "generic", although this may reduce performance.
  You can also set a semicolon-separated list of multiple different targets. See the Julia documentation for details.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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