CMake: use separate host Python interpreter and target development artifacts when cross-compiling

[ar-jan]

Description

Cross-compiling QGIS with a recent version of CMake fails at find_package(Python 3.11 REQUIRED COMPONENTS Interpreter Development), because it expects lookups for separate host Python interpreter and target development artifacts (CMP0190 is NEW, starting from CMake 4.1.).

This patch makes it so when CMP0190=NEW and CMAKE_CROSSCOMPILING are both detected, separate host Python interpreter and target development artifacts are set. Otherwise, the lookup is unchanged.

This would remove the need for patching QGIS when packaging for distributions that support cross-compiling, like I used for Void Linux here.

I've manually tested a native x86_64 build and a cross-compiled aarch64 build.

I'm admittedly not familiar with how much coordination there is around downstream packaging of QGIS, but this seems to me like a helpful change.

I think backporting would also be helpful to support packaging efforts.

AI tool usage

• [x ] AI tool(s) (Copilot, Claude, or something similar) supported my development of this PR. See our policy about AI tool use. Use of AI tools must be indicated. Failure to be honest might result in banning.

I used gpt-5.6-sol for implementing the patch.

[github-actions]

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
To execute locally, unzip the downloaded zip file and run bin\qgis-bin.exe in the extracted directory.
You might be prompted by Windows Defender click "Run anyway"
(Built from commit 8c01a50)

🍎 MacOS Qt6 builds

Download MacOS Qt6 builds of this PR for testing.
This app is not notarized, run sudo xattr -d com.apple.quarantine /Applications/QGIS*.app to avoid the warning
(Built from commit 8c01a50)