cmd/mbf: register binfmt entry for shell

This fixes --arch for shell.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/mbf/main.go

@@ -1052,6 +1052,13 @@ func main() {
 					Bind(tempdir, fhs.AbsTmp, std.BindWritable).
 					Proc(fhs.AbsProc).Dev(fhs.AbsDev, true)
 
+				if flagArch != "" && flagArch != runtime.GOARCH {
+					if entry, ok := pkg.Arch()[flagArch]; ok {
+						z.InitAsRoot = true
+						z.Binfmt = []container.BinfmtEntry{entry}
+					}
+				}
+
 				if err := z.Start(); err != nil {
 					return err
 				}