SbuildWorkflow: support a "buildd" environment variant

(From Raphaël:)

It seems likely that we will want to (be able to) schedule builds using a "buildd" variant of the "environment" (with build-essential installed).

So we need to be able to specify a variant in the workflow data and use it to lookup the environment that we are going to use. If the variant is not provided, then we use the lookup syntax without variant.

(There's no need to fallback on the default variant in case the desired variant is not available... arguably at some point we might want to be able to provide a list of variants and have the system use the first one that is available but that's for another time)

Possible name for the workflow field: environment_variant.

Related code: SbuildWorkflow._get_environment().