ctr: improve error relative shim path error msg

addresses https://github.com/containerd/containerd/issues/6464

Return an error if a runtime provided is relative.

Add context to the usage for `ctr run --runtime` indicating that
absolute path to runtime binary must be provided.

Signed-off-by: Gavin Inglis <giinglis@amazon.com>

runtime/v2/shim/util.go

@@ -85,7 +85,7 @@ func Command(ctx context.Context, config *CommandConfig) (*exec.Cmd, error) {
 func BinaryName(runtime string) string {
 	// runtime name should format like $prefix.name.version
 	parts := strings.Split(runtime, ".")
-	if len(parts) < 2 {
+	if len(parts) < 2 || parts[0] == "" {
 		return ""
 	}