flashDeps: use buildPackages to fix cross-builds

[Princemachiavelli]

Issue

Cross builds of nixosConfig containing an UEFI capsule update are currently broken due to how flashDeps are defined and used.

nix-repl> :b (nixosConfigurations.installer_minimal_cross.extendModules {
    hardware.nvidia-jetpack.enable = true; 
    modules = [{
      hardware.nvidia-jetpack.firmware.autoUpdate = true;
      hardware.nvidia-jetpack.som = "orin-nx";
     }];
}).config.system.build.toplevel

Results in exec format error due to wrong package set of flashDeps being used.

/build/.attr-0l2nkwhif96f51f4amnlf414lhl4rv9vh8iffyp431v6s28gsr90: line 24: /nix/store/g6s3cb7y0mmfa5i1s7xr5j9ma58rcwwn-coreutils-aarch64-unknown-linux-gnu-9.3/bin/cp: cannot execute binary file: Exec format error
/build/.attr-0l2nkwhif96f51f4amnlf414lhl4rv9vh8iffyp431v6s28gsr90: line 6: /nix/store/g6s3cb7y0mmfa5i1s7xr5j9ma58rcwwn-coreutils-aarch64-unknown-linux-gnu-9.3/bin/rm: cannot execute binary file: Exec format error

Changes

Use buildPackages for flashDeps so flash-script.nix uses the correct package set. As far as I can tell, there's no way to change which package set callPackage uses for passthru so directly using buildPackages seems like the best option.

The use of pkgs.buildPackages.callPackage is required for patchShebangs to set the correct interpreter in flash.sh. (Using callPackage for some reason just resulted in the shebang being left unmodified). There might be some other cause for this issue.

Testing

TODO

[jmbaur]

Some of the files in linux-firmware are symlinks, I don't think -f still works in that case. Can we also pull this change into a separate PR?

[Princemachiavelli]

Yea, the change is just so I can test this PR. I intend to remove it.

[jmbaur]

The use of pkgs.buildPackages.callPackage is required for patchShebangs

Where does this patchShebangs happen? I can't find it

[Princemachiavelli]

The use of pkgs.buildPackages.callPackage is required for patchShebangs

Where does this patchShebangs happen? I can't find it

I think it's added to the fixup in pkgs/build-support/setup-hooks/patch-shebangs.sh? I still don't understand why it's behavior would differ between pkgs.callPackage and pkgs.buildPackages unless the way it checks for build time commands fails if the architecture is different.