Try installing a default toolchain before calling cross

Author: asomers

ci/script.sh

@@ -9,6 +9,7 @@ main() {
     fi
 
     # Build debug and release targets
+    rustup default stable-x86_64-unknown-linux-gnu	# XXX does cross require a default toolchain?
     cross build --target $TARGET
     cross build --target $TARGET --release