Description of changes

Fix nixpkgs evaluation error when using nixpkgs-review on armv7l-linux:

$ nix-env --option system armv7l-linux -f /home/misuzu/nixpkgs -qaP --xml --out-path --show-trace --no-allow-import-from-derivation > /dev/null
error: attribute 'armv7l-linux' missing

       at /home/misuzu/nixpkgs/pkgs/tools/misc/grub/pvgrub_image/default.nix:24:24:

           23|     # if we include it.
           24|     grub-mkimage -O "${efiSystemsBuild.${stdenv.hostPlatform.system}.target}-xen" -c grub-bootstrap.cfg \
             |                        ^
           25|       -m memdisk.tar -o "grub-${efiSystemsBuild.${stdenv.hostPlatform.system}.target}-xen.bin" \

       … while evaluating the attribute 'buildCommand' of the derivation 'pvgrub-image'

       at /home/misuzu/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:286:7:

          285|     // (lib.optionalAttrs (attrs ? name || (attrs ? pname && attrs ? version)) {
          286|       name =
             |       ^
          287|         let

       … while querying the derivation named 'pvgrub-image'

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Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
  • armv7l-linux
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 23.05 Release Notes (or backporting 22.11 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.