[CMake] Set CMAKE_OSX_SYSROOT only when the host system is Apple macOS

[guitargeek]

Set CMAKE_OSX_SYSROOT only when the host system is Apple macO, because
otherwise we don't need it and the xcrun executable that is used to
figure out this variable is not available.

Like this, we avoid configuration failues when building ROOT is built
for Darwin in general.

This mimics what was done in CMake before version 4.0:
https://gitlab.kitware.com/search?search=xcrun&nav_source=navbar&project_id=541&group_id=415&search_code=true&repository_ref=v3.31.6

Follows up on 6bd0dba and ebc38bd.

[github-actions]

Test Results

    20 files      20 suites   3d 5h 31m 57s ⏱️
 3 063 tests  2 718 ✅   0 💤 345 ❌
59 670 runs  59 215 ✅ 110 💤 345 ❌

For more details on these failures, see this check.

Results for commit bb1035d.

[pcanal]

This mimics what was done in CMake before version 4.0:

if/since it is no longer done in 4.0, what was is replaced by or why do they not need it anymore (and we do)?

[pcanal]

This looks good to me (but see inline discussion).

[guitargeek]

if/since it is no longer done in 4.0, what was is replaced by or why do they not need it anymore (and we do)?

I'm trying to understand the PR https://gitlab.kitware.com/cmake/cmake/-/merge_requests/9977

But the more I look at at it, the less I understand why 6bd0dba was needed to begin with, because even in new CMake there is the xcrun way of determining `CMAKE_OSX_SYSROOT`` if it is not set:
https://gitlab.kitware.com/cmake/cmake/-/blob/master/Modules/Platform/Darwin-Initialize.cmake?ref_type=heads#L283

Well, I don't intend to understand everything 🙂 I was only looking for the minimal changes to make ROOT compile again in nixpkgs without patches for Darwin.