embulk --version with -X gem_home

Issue Type: Bug Report
-----------------------

* Write the following environmental information.
* OS version: macOS 13.2.1
* Java version: 1.8
* Embulk version: 0.10.48

The following command outputs `gem_home is set as a sub directory of embulk_home: /Users/user/.embulk/lib/gems` even `-X gem_home=/tmp/gem_home` set.
It's a little confusing.

```
java -jar ~/.embulk/bin/embulk-0.10.48 -X gem_home=/tmp/gem_home --version
Embulk 0.10.48
2023-04-13 14:59:48.457 +0900 [INFO] (main): m2_repo is set as a sub directory of embulk_home: /Users/user/.embulk/lib/m2/repository
2023-04-13 14:59:48.460 +0900 [INFO] (main): gem_home is set as a sub directory of embulk_home: /Users/user/.embulk/lib/gems
2023-04-13 14:59:48.460 +0900 [INFO] (main): gem_path is set empty.
2023-04-13 14:59:48.460 +0900 [DEBUG] (main): Embulk system property "default_guess_plugin" is set to: "gzip,bzip2,json,csv"
```