Fix yes no parsing

[marcoesters]

Description

Improve yes/no parsing for interactive installation with sh installers.

• Make answers case insensitive
• Accept default value for license agreement question

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

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[pseudoyim]

I built an example MinicondaX using @marcoesters's dev branch, and the default [no] now gets applied when I hit Enter, and the installer exits as expected.

I noticed two of the CI builds are failing on the example builds, but it's not clear what the causes are (here and here). Perhaps they just need a re-punch?

[jaimergp]

Seeing this code appear so often in the installer makes me wonder whether we should put it in a function, but that can be done in a different PR.

Thanks!