trace/zoekt: fix opentelemetry mode trace propagation

Created by: bobheadxi

See https://github.com/sourcegraph/sourcegraph/pull/37984, relevant bits below:

Spans that cross to zoekt-webserver - I can see the individual spans, but they aren't linked properly in Jaeger, see below (left is before, right is after)

Before	After

I suspect this is because I haven't configured the propagator correctly to work with pure OpenTracing implementations like Zoekt, but after some digging I haven't been able to figure this out. Note that spans that go between Sourcegraph services work fine:

I'm guessing https://sourcegraph.com/github.com/sourcegraph/zoekt@master/-/blob/web/trace.go?L41:6 is the thing to look at