[Backport release-25.05] qt6.qtwebengine: fix build

[nixpkgs-ci]

Bot-based backport to release-25.05, triggered by a label in #423843.

• Before merging, ensure that this backport is acceptable for the release.
  • Even as a non-committer, if you find that it is not acceptable, leave a comment.

[vcunat]

Sounds worth picking to 25.05 if it's so simple a diff (modulo reformat, sigh)
https://hydra.nixos.org/build/301703731

[vcunat]

Hmm, locally I (still) get

CMake Error at cmake/QtToolchainHelpers.cmake:21 (message):
  Can't determine darwin macosx SDK version.  Error:
Call Stack (most recent call first):
  cmake/QtToolchainHelpers.cmake:361 (get_darwin_sdk_version)
  src/core/CMakeLists.txt:316 (append_compiler_linker_sdk_setup)

No idea what's wrong here, and I don't have mental capacity for this, I'm afraid.

[vcunat]

Maybe @Prince213 will know, off the top of his head.

[Prince213]

Hi @vcunat, as I've said in #415929 (comment), #423843 will only fix 6.9.0 and #424034 is also required to fix 6.9.1.

[Prince213]

#424794

[vcunat]

Aah, I got tangled in this. Thanks.