distribution: SA4021: x = append(y) is equivalent to x = y (staticcheck)

```
distribution/push_v2_test.go:552:29: SA4021: x = append(y) is equivalent to x = y (staticcheck)
	return nil, errcode.Errors(append([]error{errcode.ErrorCodeUnauthorized.WithMessage("unauthorized")}))
	                           ^
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

distribution/push_v2_test.go

@@ -549,7 +549,7 @@ type mockBlobStoreWithCreate struct {
 }
 
 func (blob *mockBlobStoreWithCreate) Create(ctx context.Context, options ...distribution.BlobCreateOption) (distribution.BlobWriter, error) {
-	return nil, errcode.Errors(append([]error{errcode.ErrorCodeUnauthorized.WithMessage("unauthorized")}))
+	return nil, errcode.Errors([]error{errcode.ErrorCodeUnauthorized.WithMessage("unauthorized")})
 }
 
 type mockRepoWithBlob struct {