Fix jwt -show

Some time ago (was lazy to dig history) jwt.Parse was changed to require
non-nil keyFunc.

This broke `jwt -show` command that prints header and claims and
does not validate the token:
```
$ echo $TOKEN | jwt -show -
Error: malformed token: token is unverifiable: no keyfunc was provided
```

This change fixes `jwt -show`.

cmd/jwt/main.go

@@ -273,7 +273,7 @@ func showToken() error {
 		fmt.Fprintf(os.Stderr, "Token len: %v bytes\n", len(tokData))
 	}
 
-	token, err := jwt.Parse(string(tokData), nil)
+	token, _, err := jwt.NewParser().ParseUnverified(string(tokData), make(jwt.MapClaims))
 	if err != nil {
 		return fmt.Errorf("malformed token: %w", err)
 	}