conda: Set QT_HOST_PATH in conda's build.sh CMake template for osx-arm64 compat

[traversaro]

Currently conda packages build on osx-arm64 fail with error:

2024-10-03T20:47:09.6134360Z CMake Error at /Users/runner/miniconda3/envs/test/conda-bld/walking-controllers_1727988336360/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold/lib/cmake/Qt6/QtPublicDependencyHelpers.cmake:258 (message):
2024-10-03T20:47:09.6136720Z   To use a cross-compiled Qt, please set the QT_HOST_PATH cache variable to
2024-10-03T20:47:09.6137370Z   the location of your host Qt installation.
2024-10-03T20:47:09.6137790Z Call Stack (most recent call first):

This is due to conda-forge/qt-main-feedstock#273 . As a workaround, we add -DQT_HOST_PATH=$PREFIX in the build.sh template for CMake projects.