fix: prevent MiniClusterCtx race conditions in command shutdown

Capture global MiniClusterCtx into local variables before goroutine/select
evaluation to prevent nil dereference/data race when context is reset to nil
after nil check. Applied to filer, master, volume, and s3 commands.

weed/command/s3.go

@@ -408,8 +408,9 @@ func (s3opt *S3Options) startS3Server() bool {
 			}
 			httpS := newHttpServer(router, tlsConfig)
 			if MiniClusterCtx != nil {
+				ctx := MiniClusterCtx
 				go func() {
-					<-MiniClusterCtx.Done()
+					<-ctx.Done()
 					httpS.Shutdown(context.Background())
 					grpcS.Stop()
 				}()