Description of changes

Fixes a small issue with luksroot's implementation for systemd-stage-1 when keyFileSize (int) is used in a string without conversion.

cc: @dasJ

without this PR:

error: cannot coerce an integer to a string

       at /nix/store/yr20q5ja0shqbgcp3c229h0anp5bqw6x-source/nixos/modules/system/boot/luksroot.nix:485:57:

          484|       ++ optional (v.keyFileOffset != null) "keyfile-offset=${v.keyFileOffset}"
          485|       ++ optional (v.keyFileSize != null) "keyfile-size=${v.keyFileSize}"
             |                                                         ^
          486|     ;
(use '--show-trace' to show detailed location information)

(and this PR does indeed then fix it)

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• 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/)
• 22.11 Release Notes (or backporting 22.05 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.