This function was broken, because it took ownership of the buffer on
error *sometimes*, in a way which the caller could not tell. Rather than
trying to be clever, it's easier to just follow the same interface as
all other `addFilePost` methods, and not take ownership of the path.

This is a breaking change. The next commits will apply it to the
compiler, which is the only user of this function in the ziglang/zig
repository.