Fail the build if global.json was updated. (#20983)

While not strictly necessary for a successful build, the API diff will
fail unless we have the correct global.json contents checked in.

Makefile

@@ -75,6 +75,12 @@ global.json: $(TOP)/dotnet.config Makefile $(GIT_DIRECTORY)/HEAD $(GIT_DIRECTORY
 		printf "  \"sdk\": {\n    \"version\": \"$(DOTNET_VERSION)\"\n  }\n" >> $@; \
 		printf "}\n" >> $@
 
+install-hook::
+	$(Q) if ! git diff --exit-code global.json; then \
+		echo "Error: global.json has changed: please commit the changes."; \
+		exit 1; \
+	fi
+
 install-hook::
 	@$(MAKE) check-permissions
 ifdef INCLUDE_IOS