pkg/util/xlog: fix AddPrefix not updating existing prefix due to range value copy (#5206)

In AddPrefix, the loop `for _, p := range l.prefixes` creates a copy
of each element. Assignments to p.Value and p.Priority only modify
the local copy, not the original slice element, causing updates to
existing prefixes to be silently lost.

This affects client/service.go where AddPrefix is called with
Name:"runID" on reconnection — the old runID value would persist
in log output instead of being updated to the new one.

Fix by using index-based access `l.prefixes[i]` to modify the
original slice element, and add break since prefix names are unique.