patch_manager: Prevent use of a dangling pointer within PatchRomFS

fmt::format() returns a std::string instance by value, so calling
.c_str() on it here is equivalent to doing:

auto* ptr = std::string{}.c_str();

The data being pointed to isn't guaranteed to actually be valid anymore
after that expression ends. Instead, we can just take the string as is,
and provide the necessary formatting parameters.