Builds fail on macOS without `--xcode_version`

[sgielen]

$ bonanza_bazel build --config=bonanza //cmd/bb_storage
INFO: Scanning module sources
INFO: Uploading module sources
INFO: Performing build 91RwhadckHfvmPGfL82n9p_4-Mmr98phw3kyfeJ2ScFIAQAGAQDMKw.EYowpAvj6Mt6zaT3umtVHg
ERROR: Traceback (most recent key last):
[...]
ERROR: Failed to perform build: rpc error: code = FailedPrecondition desc = Failed to run command: Attempted to use Xcode installation with version "None", while only [] are supported

However, with --xcode_version:

$ bonanza_bazel build --config=bonanza --xcode_version=$(xcode-select -p) //cmd/bb_storage
INFO: Scanning module sources
INFO: Uploading module sources
INFO: Performing build UmdAoUIe3gJKUqaMiEKT4bIAQPvnZZiYmGXE3d1bTv-oAQAGAQDMKw.zzla4H6HYbcROWbChOk-Ew
INFO: 🏁 131531   🚗💨 100   🚦 101   🚧 4068
[...]