build: add temporary workaround for Rust on macOS

Temporarily install the x64 Rust target on macOS at build time.
To be removed when the target is installed at the macOS VM level.

Signed-off-by: Richard Lau <richard.lau@ibm.com>

Author: richardlau

Makefile

@@ -1175,6 +1175,8 @@ endif
 			| sed -E "s/\\{npmversion\\}/$(NPMVERSION)/g"  \
 		>$(MACOSOUTDIR)/installer/productbuild/Resources/$$lang/conclusion.html ; \
 	done
+	# TODO(richardlau) remove once target is added to the underlying macOS VMs. 
+	rustup target add x86_64-apple-darwin
 	CC_host="cc -arch x86_64" CXX_host="c++ -arch x86_64"  \
 	CC_target="cc -arch x86_64" CXX_target="c++ -arch x86_64" \
 	CC="cc -arch x86_64" CXX="c++ -arch x86_64" $(PYTHON) ./configure \