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
Fixes: #20023