fix: variant strict typing

[wolfv]

There are some annyoing things going on:

• conda-build converts true to 'true', and 5 to '5'
• rattler-build converts it back to boolean
• somewhere in between conda-smithy gets confused

I think we should be strict about the parsing in rattler-build and quoted strings should be strings when they come from variants.yaml.

The behavior of conda-smithy means that we cannot ship the deprecation warning though (#1410).