do the swift-models build with the right flag when using the stock toolchain (#1163)

Author: marcrasi

Dockerfile

@@ -76,9 +76,11 @@ RUN git clone https://github.com/deepmind/open_spiel.git
 
 WORKDIR /swift-models
 
-RUN /swift-tensorflow-toolchain/usr/bin/swift build
+RUN /swift-tensorflow-toolchain/usr/bin/swift build \
+  $([ "$TENSORFLOW_USE_STANDARD_TOOLCHAIN" = "YES" ] && echo "-Xswiftc -D -Xswiftc TENSORFLOW_USE_STANDARD_TOOLCHAIN")
 # Swift Numerics in release mode does not currently build, which prevents the use of swift-models
-# RUN /swift-tensorflow-toolchain/usr/bin/swift build -c release
+# RUN /swift-tensorflow-toolchain/usr/bin/swift build -c release \
+#   $([ "$TENSORFLOW_USE_STANDARD_TOOLCHAIN" = "YES" ] && echo "-Xswiftc -D -Xswiftc TENSORFLOW_USE_STANDARD_TOOLCHAIN")
 
 WORKDIR /fastai_dev/swift/FastaiNotebook_11_imagenette