[#1341] .golangci.yml: Replace exportloopref with copyloopvar

exportloopref is deprecated.
gopatch:
```
@@
var index, value identifier
var slice expression
@@
for index, value := range slice {
...
-value := value
...
}

@@
var index, value identifier
var slice expression
@@
for index, value := range slice {
...
-index := index
...
}

@@
var value identifier
var channel expression
@@
for value := range channel {
...
-value := value
...
}
```

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/services/tree/sync_test.go

@@ -51,8 +51,6 @@ func Test_mergeOperationStreams(t *testing.T) {
 
 			// generate and put values to all chans
 			for i, ch := range nodeOpChans {
-				i := i
-				ch := ch
 				go func() {
 					for _, tm := range tt.opTimes[i] {
 						op := &pilorama.Move{}