lib/log: add caller skip to point to line of Scope creation

Created by: bobheadxi

I notice that the debug lines we print to document information about logger scopes will point to lib/log instead of the line of scope creation, so we add a caller skip to correct this.

This doesn't remediate the issue for observation.Context.Operation's usage of .Scoped but can address in a follow-up (https://sourcegraph.com/github.com/sourcegraph/sourcegraph@17100e5895c086e0387c0f5e7416ca5c6be8b24c/-/blob/internal/observation/observation.go?L144-152), it's a bit clunky ergonomics-wise

Test plan

sg start --debug=enterprise-frontend