Pessimistic logic in detection of MacOS SDKROOT

[m-hilgendorf]

This code appears to be overly pessimistic when detecting the SDKROOT environment variable, specifically if the SDK has been relocated to a different directory that does not contain the string "MacOSX."

This causes build failures downstream when a user explicitly sets SDKROOT before invoking a cargo build, and upstream dependencies fail with cryptic xcrun errors.