setup.sh: use -exec rather than -execdir

[ghost]

Note: this is untested. I will undraft when I have verified it. That will take a while (global rebuild). nix build abuild completed; rebuilding the rest of my userspace (should be done by morning)

Description of changes

Busybox's find does not support -execdir, so let's uses -exec instead, in the spirit of requiring the minimum features needed for the task. The benefit of -execdir over -exec is robustness against TOCTOU (Time Of Check Time Of Use) attacks, which I believe are not a concern here.

Some packages (e.g. abuild) put busybox into their nativeBuildInputs, which leads to setup.sh using busybox find rather than $findutils/bin/find (because busybox find is earlier in the $PATH). This PR will fix those packages, although it really isn't a good thing if they are inadvertently changing which stdenv-tools are being used by setup.sh. This PR should not be interpreted as encouraging that sort of thing.

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.

[Mindavi]

This looks quite safe to me.