Replace absolute path with relative path after generating xcodeproj

Makefile

@@ -16,6 +16,8 @@ project:
 
 project-carthage:
 	swift package generate-xcodeproj --output SwiftGRPC-Carthage.xcodeproj
+	@sed -i '' -e "s|$(PWD)|..|g" SwiftGRPC-Carthage.xcodeproj/project.pbxproj
+	@sed -i '' -e "s|$(PWD)|../../..|g" SwiftGRPC-Carthage.xcodeproj/GeneratedModuleMap/BoringSSL/module.modulemap
 	@ruby fix-project-settings.rb SwiftGRPC-Carthage.xcodeproj || echo "xcodeproj ('sudo gem install xcodeproj') is required in order to generate the Carthage-compatible project!"
 	@ruby patch-carthage-project.rb SwiftGRPC-Carthage.xcodeproj || echo "xcodeproj ('sudo gem install xcodeproj') is required in order to generate the Carthage-compatible project!"