Conditionals not supported in element overrides

Summary

Trying to use a conditional inside an element override, I get an error: Value of '(?)' is not of the expected type 'str'

Steps to reproduce

Here is a small project.conf to reproduce

name: testcase

options:
  arch:
    type: arch
    description: architecture
    values: [i686, x86_64]

elements:
  autotools:
    variables:
      (?):
      - arch == 'i686':
          conf-global: --host=i686-unknown-linux-gnu
      - arch == 'x86_64':
          conf-global: --host=x86_64-unknown-linux-gnu

What is the current bug behavior?

project.conf [line 13 column 6]: Value of '(?)' is not of the expected type 'str'

What is the expected correct behavior?

It should work

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.2

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