Setting a common CMAKE_STAGING_PREFIX breaks build rpaths of non-qtbase repositories

Upstream issue https://gitlab.kitware.com/cmake/cmake/-/issues/23421

Scenario:

Desktop Linux / macOS

qtbase/configure -extprefix /tmp/sysroot -prefix /usr && ninja instlal
qt-configure-module qtshadertools && ninja install
qt-configure-module qtdeclarative && ninja

qtdeclarative will fail to build with

 Automatic QML type registration for target Qml
FAILED: src/qml/qml_qmltyperegistrations.cpp

dyld: Library not loaded: @rpath/libQt6Core_debug.6.dylib
  Referenced from: /qtdeclarative_build/libexec/qmltyperegistrar
  Reason: image not found
/bin/sh: line 1:  2493 Abort trap: 6 qtdeclarative_build/libexec/qmltyperegistrar

CMake has logic to replace any rpath that it encounters that starts with CMAKE_STAGING_PREFIX, with CMAKE_INSTALL_PREFIX.

I believe this makes sense for install rpaths, but not build rpaths.

qmltyperegistrar depends on QtCore, so I expect the /tmp/sysroot/lib rpath to be present, so that it successfully loads Core, but it's rewritten to /usr/lib, which has no Qt on my machine, and fails to run.

All binaries / libraries outside of qtbase are affected, but the issue mostly surfaces with tools that actually need to run during the build.

Upon installation, all the build rpaths are rewritten to correct install rpaths, so installed binaries are not affected.

Cross-builds in the CI (e.g. qemu / yocto) are not affected, because they use host tools, and none of our host CI builds use CMAKE_STAGING_PREFIX.

Top-level builds are not affected because CMake will not embed any rpaths to the staging area, because there are no packages / libraries loaded from there at build time.

Non-prefix staging builds also have wrong rpaths, aka

mkdir ~/qtbase_build && cd ~/qtbase_build
~/qtbase/configure -extprefix ~/qtbase_build -prefix /usr

but they usually don't fail because qt_apply_rpaths adds additional build time relative rpaths with $ORIGIN/@loader_path , which are not rewritten.
If relative rpaths are disabled or unsupported, the build still fails.

Also, for standalone tests, we explicitly work around the issue by setting CMAKE_STAGING_PREFIX of the tests subdirectory to an empty string, to avoid CMake rewriting the rpaths. It's safe to do for tests, because we never intend to install tests.
https://codereview.qt-project.org/c/qt/qtbase/+/318708/3/cmake/QtBuildInternals/QtBuildInternalsConfig.cmake