Xcode 12 doesn't allow nested builds within the same build directory. That means we can no longer do an install by building the install target when IOS_INSTALL_COMBINED is true. We can, however, still do an install by running the cmake_install.cmake script or executing cmake --install, since there is no outer build and therefore the associated SDK can be built as a sub-build.
The non-build methods previously didn't work when IOS_INSTALL_COMBINED was true because the generated install script and the CMakeIOSInstallCombined script both made certain assumptions that relied on being part of a build. Those assumptions are now removed. A side-effect of this work is that cpack now also works from the command line when IOS_INSTALL_COMBINED is true.
Relates: #21282 (closed)
Fixes: #20023 (closed)
Backport: release