cmake.eclass sets CMAKE_TOOLCHAIN_FILE to a temporary file, but
Qt expects it to not be temporary. Projects shouldn't be loading
gentoo's toolchain file outside ebuilds either way.

https://bugs.gentoo.org/984102
--- a/cmake/QtToolchainHelpers.cmake
+++ b/cmake/QtToolchainHelpers.cmake
@@ -7,3 +7,3 @@
 function(qt_internal_create_toolchain_file)
-    if(CMAKE_TOOLCHAIN_FILE)
+    if(0)
         # For OHOS, CMAKE_TOOLCHAIN_FILE is replaced with a build-dir wrapper during configure.