Do not use just-built CMake to install itself when cross-compiling (#15248)

The special case added by commit v2.4.0~3051 (When building CMake
itself, use the new cmake to install so that the current cmake can be
overwritten, 2004-06-09) does not make sense when cross-compiling.
Merge topic 'no-cmake-self-install-when-cross-compiling' into next

15f55cca Do not use just-built CMake to install itself when cross-compiling (#15248)