pkg/skel: missing env var log lines appear in stderr

Previously, the log lines appeared in stdout before the JSON encoding of
the error message.  That would break JSON parsing of stdout.  Instead, we use
stderr for these unstructured logs, consistent with the CNI spec.

pkg/skel/skel.go

@@ -40,6 +40,7 @@ type CmdArgs struct {
 type dispatcher struct {
 	Getenv func(string) string
 	Stdin  io.Reader
+	Stderr io.Writer
 }
 
 type reqForCmdEntry map[string]bool
@@ -106,8 +107,7 @@ func (t *dispatcher) getCmdArgsFromEnv() (string, *CmdArgs, error) {
 	for _, v := range vars {
 		*v.val = t.Getenv(v.name)
 		if v.reqForCmd[cmd] && *v.val == "" {
-			log.Printf("%v env variable missing", v.name)
-			// TODO: test this logging ^^^  and log to stderr instead of stdout
+			fmt.Fprintf(t.Stderr, "%v env variable missing\n", v.name)
 			argsMissing = true
 		}
 	}
@@ -172,6 +172,7 @@ func PluginMain(cmdAdd, cmdDel func(_ *CmdArgs) error) {
 	caller := dispatcher{
 		Getenv: os.Getenv,
 		Stdin:  os.Stdin,
+		Stderr: os.Stderr,
 	}
 
 	err := caller.pluginMain(cmdAdd, cmdDel)