ctr: print deprecation warnings on every invocation

Print deprecation warnings on any ctr command, as users won't notice the
deprecations until we actually remove the deprecated features.

The warnings can be suppressed by setting
`CONTAINERD_SUPPRESS_DEPRECATION_WARNINGS=1`.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/ctr/commands/deprecations/deprecations.go

@@ -47,6 +47,9 @@ var listCommand = cli.Command{
 		},
 	},
 	Action: func(context *cli.Context) error {
+		// Suppress automatic warnings, since we print the warnings by ourselves.
+		os.Setenv("CONTAINERD_SUPPRESS_DEPRECATION_WARNINGS", "1")
+
 		client, ctx, cancel, err := commands.NewClient(context)
 		if err != nil {
 			return err