ci: ignore err-check lintings for io.Copy

.golangci.yml

@@ -1,5 +1,7 @@
 linters-settings:
   errcheck:
     exclude-functions:
+      # Don't flag lines such as "io.Copy(io.Discard, resp.Body)".
+      - io.Copy
       # Never check for logger errors.
       - (github.com/go-kit/log.Logger).Log