cli/command/system: fix "unused-receiver" linting

cli/command/system/dial_stdio.go:113:7: unused-receiver: method receiver 'x' is not referenced in method's body, consider removing or renaming it as _ (revive)
    func (x *nopCloseReader) CloseRead() error {
          ^

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

cli/command/system/dial_stdio.go

@@ -110,7 +110,7 @@ type nopCloseReader struct {
 	halfReadWriteCloser
 }
 
-func (x *nopCloseReader) CloseRead() error {
+func (*nopCloseReader) CloseRead() error {
 	return nil
 }