Native packages cannot be used during cross-compiling. However, Qt still unconditionally tries to find them, which causes issues in some cases, such as when cross-compiling from macOS to Windows.
This PR explicitly disables this unnecessary Qt behaviour.
Fixes #32346.
Here is a full workflow on my macOS Sequoia 15.4.1 (Intel):
0% brew install make cmake ninja mingw-w64 nsis
1% gmake -C depends -j 10 HOST=x86_64-w64-mingw32
2% cmake -B build --toolchain depends/x86_64-w64-mingw32/toolchain.cmake
3% cmake --build build -j 10 -t deploy