ollama: mark cross as broken, fix cmake invocation

[FliegendeWurst]

By default $cmakeFlags is empty, if cross-compiling it isn't. This ensures the build fails loudly even if meta.broken is ignored.

Not tested: whether enabling rocm or cuda would add undesired flags to $cmakeFlags.

Things done

• Built on platform(s)
  • x86_64-linux (acceleration = null, $cmakeFlags empty, so no change)
  • 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.

[Aleksanaa]

By default $cmakeFlags is empty, if cross-compiling it isn't. This ensures the build fails loudly even if meta.broken is ignored.

What are you trying to achieve with that? Are there any hidden problems with the output of cross compilation at present?

[FliegendeWurst]

Yes, it does CPU feature detection using the builder CPU. And $cmakeFlags are required to successfully cross-compile at all, I think.
(At present it does not cross compile to e.g. aarch64: log.)