Fix macOS makefile search path

Makefile

@@ -76,7 +76,7 @@ docker-test:
 # Test with the homebrew version of icu4c on macOS with static linking (the default way of linking for distribution on Apple platforms)
 macos-test:
 	brew install icu4c
-	RUST_ICU_LINK_SEARCH_DIR=$(brew --prefix)/opt/icu4c/lib \
+	RUST_ICU_LINK_SEARCH_DIR="$(shell brew --prefix)/opt/icu4c/lib" \
 	RUST_ICU_MAJOR_VERSION_NUMBER=${RUST_ICU_MAJOR_VERSION_NUMBER} \
 	cargo test --no-default-features --features=icu_version_64_plus,icu_version_67_plus,icu_version_68_plus,icu_version_in_env,renaming,static
 .PHONY: macos-test