Convert CLI to urfave v2

Followed the Migration Guide at https://cli.urfave.org/migrate-v1-to-v2/
The major changes not pointed out in the migration guide are:
- context.Args() no longer produces a []slice, so context.Args().Slice()
  in substitued
- All cli.Global***** are deprecated (the migration guide is somewhat
  unclear on this)

Signed-off-by: Derek Nola <derek.nola@suse.com>

Vendor in urfave cli/v2

Signed-off-by: Derek Nola <derek.nola@suse.com>

Fix NewStringSlice calls

Signed-off-by: Derek Nola <derek.nola@suse.com>

cmd/ctr/commands/images/export.go

@@ -22,7 +22,7 @@ import (
 	"io"
 	"os"
 
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 
 	"github.com/containerd/containerd/v2/cmd/ctr/commands"
 	"github.com/containerd/containerd/v2/core/images/archive"
@@ -32,7 +32,7 @@ import (
 	"github.com/containerd/platforms"
 )
 
-var exportCommand = cli.Command{
+var exportCommand = &cli.Command{
 	Name:      "export",
 	Usage:     "Export images",
 	ArgsUsage: "[flags] <out> <image> ...",
@@ -44,24 +44,24 @@ Use '--platform' to define the output platform.
 When '--all-platforms' is given all images in a manifest list must be available.
 `,
 	Flags: []cli.Flag{
-		cli.BoolFlag{
+		&cli.BoolFlag{
 			Name:  "skip-manifest-json",
 			Usage: "Do not add Docker compatible manifest.json to archive",
 		},
-		cli.BoolFlag{
+		&cli.BoolFlag{
 			Name:  "skip-non-distributable",
 			Usage: "Do not add non-distributable blobs such as Windows layers to archive",
 		},
-		cli.StringSliceFlag{
+		&cli.StringSliceFlag{
 			Name:  "platform",
 			Usage: "Pull content from a specific platform",
-			Value: &cli.StringSlice{},
+			Value: cli.NewStringSlice(),
 		},
-		cli.BoolFlag{
+		&cli.BoolFlag{
 			Name:  "all-platforms",
 			Usage: "Exports content from all platforms",
 		},
-		cli.BoolFlag{
+		&cli.BoolFlag{
 			Name:  "local",
 			Usage: "Run export locally rather than through transfer API",
 		},