[rubygems/rubygems] Set $0 to exe when running `gem exec` to fix name in CLI output

The $0 value is used in many CLI libraries to determine the name of the
application, when displaying help and error messages.

Without setting this value, it defaults to `gem` which can be confusing.

Before:

```
$ gem exec kamal help
Commands:
  gem accessory           # Manage accessories (db/redis/search)
  gem app                 # Manage application
  gem audit               # Show audit log from servers
  gem build               # Build application image
  gem config              # Show combined config (including secrets!)
  gem deploy              # Deploy app to servers
  gem details             # Show details about all containers
  gem docs [SECTION]      # Show Kamal configuration documentation
  gem help [COMMAND]      # Describe available commands or one specific command
  gem init                # Create config stub in config/deploy.yml and secrets stub in .kamal
  gem lock                # Manage the deploy lock
  gem proxy               # Manage kamal-proxy
  gem prune               # Prune old application images and containers
  gem redeploy            # Deploy app to servers without bootstrapping servers, starting kamal-proxy, pruning, and registry login
  gem registry            # Login and -out of the image registry
  gem remove              # Remove kamal-proxy, app, accessories, and registry session from servers
  gem rollback [VERSION]  # Rollback app to VERSION
  gem secrets             # Helpers for extracting secrets
  gem server              # Bootstrap servers with curl and Docker
  gem setup               # Setup all accessories, push the env, and deploy app to servers
  gem upgrade             # Upgrade from Kamal 1.x to 2.0
  gem version             # Show Kamal version
```

After:

```
$ gem exec kamal help
Commands:
  kamal accessory           # Manage accessories (db/redis/search)
  kamal app                 # Manage application
  kamal audit               # Show audit log from servers
  kamal build               # Build application image
  kamal config              # Show combined config (including secrets!)
  kamal deploy              # Deploy app to servers
  kamal details             # Show details about all containers
  kamal docs [SECTION]      # Show Kamal configuration documentation
  kamal help [COMMAND]      # Describe available commands or one specific command
  kamal init                # Create config stub in config/deploy.yml and secrets stub in .kamal
  kamal lock                # Manage the deploy lock
  kamal proxy               # Manage kamal-proxy
  kamal prune               # Prune old application images and containers
  kamal redeploy            # Deploy app to servers without bootstrapping servers, starting kamal-proxy, pruning, and registry login
  kamal registry            # Login and -out of the image registry
  kamal remove              # Remove kamal-proxy, app, accessories, and registry session from servers
  kamal rollback [VERSION]  # Rollback app to VERSION
  kamal secrets             # Helpers for extracting secrets
  kamal server              # Bootstrap servers with curl and Docker
  kamal setup               # Setup all accessories, push the env, and deploy app to servers
  kamal upgrade             # Upgrade from Kamal 1.x to 2.0
  kamal version             # Show Kamal version
```

https://github.com/rubygems/rubygems/commit/4fd060b96d

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