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](https://luci-milo.appspot.com/raw/build/logs.chromium.org/flutter/led/vashworth_google.com/04e485a0b152a0720f5e561266f7a6e4fb64fc76227fcacc95b67486ae2771e7/+/build.proto)
* [Post-fix success](https://luci-milo.appspot.com/raw/build/logs.chromium.org/flutter/led/vashworth_google.com/d454a3e181e1a97692bdc1fcc197738fe04e4acf1cb20026fd040fd78513f3b0/+/build.proto)

Fixes https://github.com/flutter/flutter/issues/132755.