Starting in Xcode 15, when building macOS, DT_TOOLCHAIN_DIR cannot be used to evaluate LD_RUNPATH_SEARCH_PATHS or LIBRARY_SEARCH_PATHS. `xcodebuild` error message recommend using TOOLCHAIN_DIR instead.
Since Xcode 15 isn't in CI, I tested it in a one-off `led` test:
* [Pre-fix failure](04e485a0b1/+/build.proto)
* [Post-fix success](d454a3e181/+/build.proto)
Fixes https://github.com/flutter/flutter/issues/132755.