xcbuild: add SDKROOT by default to the wrappers

[copumpkin]

This fixes #30269 and lets us do things like xcrun -find cc, for example.

cc @matthewbauer

Motivation for this change

#30269

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[copumpkin]

I'll probably also upgrade xcbuild later. There are no new official releases but the current one has a nasty bug where it runs build steps out of order and master has a fix for that, so I'm just going to build that unless you object.

[matthewbauer]

Looks good! Keep track of some of the darwin.* stuff after this is merged because they can sometimes break from changes like these.

[copumpkin]

Thanks!