[v0.11] make tracing socket forward error non-fatal

Signed-off-by: Tonis Tiigi <[email protected]>
(cherry picked from commit c5143c0)

Author: tonistiigi

cmd/buildkitd/main.go

@@ -663,7 +663,8 @@ func newController(c *cli.Context, cfg *config.Config) (*control.Controller, err
 	if tc != nil {
 		traceSocket = traceSocketPath(cfg.Root)
 		if err := runTraceController(traceSocket, tc); err != nil {
-			return nil, err
+			logrus.Warnf("failed set up otel-grpc controller: %v", err)
+			traceSocket = ""
 		}
 	}