apptainer.gpuChecks.saxpy: fix the eval

[trofi]

Without the change the eval fails as:

$ nix build --no-link -f. apptainer.gpuChecks.saxpy
error:
       error: attribute 'image-saxpy' missing
       at pkgs/applications/virtualization/singularity/generic.nix:303:72:
          302|                 ''
          303|                   ${lib.getExe finalAttrs.finalPackage} exec --nv $@ ${finalAttrs.passthru.tests.image-saxpy} saxpy
             |                                                                        ^
          304|                 '';

Description of changes

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/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 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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[SomeoneSerge]

Thank you @trofi! I probably missed this because ofborg only evaluates outPath and passthru.tests...