fix Launchd export - replace $PORT in program args

replace $PORT with `engine.port_for(process, num)` in ProgramArguments of Launchd export.

## problem

export launchd .plist file

    % foreman export launchd ~/Library/LaunchAgents/ --app test-app --port 6100

`$PORT` in ProgramArguments will not be 6100 on Launchd.
It will be string "$PORT".

```xml
<key>EnvironmentVariables</key>
<dict>
    <key>PORT</key>
    <string>6100</string>
</dict>
<key>ProgramArguments</key>
<array>
    <string>bundle</string>
    <string>exec</string>
    <string>rackup</string>
    <string>config.ru</string>
    <string>-p</string>
    <string>$PORT</string>
</array>
```

## fix
replace $PORT with `engine.port_for(process, num)`

```xml
<key>EnvironmentVariables</key>
<dict>
    <key>PORT</key>
    <string>6100</string>
</dict>
<key>ProgramArguments</key>
<array>
    <string>bundle</string>
    <string>exec</string>
    <string>rackup</string>
    <string>config.ru</string>
    <string>-p</string>
    <string>6100</string>
</array>
```