Fix a CLI encConfig error

[ant1597]

When running the following command

squoosh-cli --mozjpeg -d .\out\ .\in\

Node.js returns a TypeError:

TypeError: encParam.toLowerCase is not a function
    at Command.processFiles (file:///C:/Users/Admin/AppData/Roaming/npm/node_modules/@squoosh/cli/src/index.js:172:18)

System: Windows 10
Node.js version: 16.13.1

I have not digged into this issue, but according to a Stack Overflow discussion on a similar issue in React.js, here should have a toString() method before toLowercase() method. I hope I got it correctly.

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[ziemek99]

Should be closed because of #1321.