macOS SDK directory changed

Starting with conda-smithy 3.54.0, the generated build scripts for macOS will no longer use the system SDK directory for downloading the SDK versions we require, but will use a dedicated /opt/conda-sdks directory instead. Users performing local builds will need to choose a writable directory, and provide the path to it via the environment variable OSX_SDK_DIR.

This change may result in some build systems, particularly CMake, storing paths to this temporary build directory in installed metadata. Feedstocks will need to substitute the stored paths with path-agnostic solutions (for example, see substitutions in openpmd-api-feedstock) or the correct sysroot paths (for example, see substitutions in cartographer-feedstock).