fix(depends): tell the consuming build where the host Qt is

A cross Qt carrying qtdeclarative makes find_package(Qt6) demand a host Qt,
because QML needs host-side tools: qmlcachegen, qmlimportscanner and
qmltyperegistrar. depends already builds one. native_qt installs it to
native/qt-host, and qt.mk passes that path to the cross Qt's own configure.

What nobody passed it to was wowlet. Qt6Config re-runs the host-path setup when
our project calls find_package, finds nothing, and stops with "To use a
cross-compiled Qt, please set the QT_HOST_PATH cache variable". The Windows
cross build has failed that way on every tag since qtdeclarative landed in
June, which is why 5.1.4 shipped macOS and Linux but no exe.

Stating it in the toolchain rather than in the workflow fixes every cross
target and anyone building locally through depends, and the built-cache key
already hashes this file, so the stale mingw tree is rebuilt rather than
restored without the host Qt.

contrib/depends/toolchain.cmake.in

@@ -43,6 +43,15 @@ SET(absl_ROOT @prefix@)
 SET(NATIVE_BIN_PATH @prefix@/native/bin CACHE FILEPATH "path to native binaries" FORCE)
 SET(LRELEASE_PATH @prefix@/native/bin CACHE FILEPATH "path to lrelease" FORCE)
 
+# A cross Qt that carries qtdeclarative makes find_package(Qt6) demand a HOST Qt,
+# because QML needs host-side tools (qmlcachegen, qmlimportscanner,
+# qmltyperegistrar). depends already builds one: native_qt installs to
+# native/qt-host and qt.mk hands that path to the cross Qt's own configure. Our
+# build never heard about it, so Qt6Config refused with "please set the
+# QT_HOST_PATH cache variable" and the Windows cross build has failed since
+# qtdeclarative landed. Same path, stated once more for the consuming project.
+SET(QT_HOST_PATH @prefix@/native/qt-host CACHE PATH "host Qt for cross QML tooling")
+
 if(NOT CMAKE_SYSTEM_NAME STREQUAL "Android")
 SET(LIBUNWIND_INCLUDE_DIR @prefix@/include)
 SET(LIBUNWIND_LIBRARIES @prefix@/lib/libunwind.a)