macOS SDK directory changed

Starting with conda-smithy 3.53.4, the build scripts will no longer use the system macOS SDK directory by default. The CI systems will use a dedicated /opt/conda-sdks directory, while users performing local builds will need to choose a writable directory, and provide the path to it via OSX_SDK_DIR environment variable.

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.