fix: apply staticcheck suggestions (#262)

Preliminary work done as part of https://github.com/ooni/probe/issues/1299

internal/engine/experimentbuilder.go

@@ -193,7 +193,7 @@ func canonicalizeExperimentName(name string) string {
 }
 
 func newExperimentBuilder(session *Session, name string) (*ExperimentBuilder, error) {
-	factory, _ := experimentsByName[canonicalizeExperimentName(name)]
+	factory := experimentsByName[canonicalizeExperimentName(name)]
 	if factory == nil {
 		return nil, fmt.Errorf("no such experiment: %s", name)
 	}