Update gomod and vendor

vendor/github.com/moby/buildkit/util/tracing/multispan.go

@@ -0,0 +1,22 @@
+package tracing
+
+import (
+	opentracing "github.com/opentracing/opentracing-go"
+)
+
+// MultiSpan allows shared tracing to multiple spans.
+// TODO: This is a temporary solution and doesn't really support shared tracing yet. Instead the first always wins.
+
+type MultiSpan struct {
+	opentracing.Span
+}
+
+func NewMultiSpan() *MultiSpan {
+	return &MultiSpan{}
+}
+
+func (ms *MultiSpan) Add(s opentracing.Span) {
+	if ms.Span == nil {
+		ms.Span = s
+	}
+}