ci: Revert packaging to macOS 15.5 SDK to restore cmake-gui rendering

Since commit 965a12cb8a (ci: update macOS jobs to use Xcode 26.0,
2025-09-18, v4.1.2~9^2) and commit 9d302ecd47 (ci: update macOS jobs to
use Xcode 26.0 in CMake 3.31 branch, 2025-10-07) our macOS 10.13+
packages are built using the macOS 26 SDK, with which Qt 5.15.2 does not
render buttons in `cmake-gui` correctly.  Revert to an older macOS SDK
to avoid the problem until we update our Qt version.

Fixes: #27325

.gitlab/ci/env_macos_package.sh

@@ -0,0 +1 @@
+. .gitlab/ci/macos-env.sh