Allow variable substitution in `variables` section

[finestructure]

For example, this should work if TEST_ID is an environment variable:

variables:
  email: foo-${TEST_ID}@bar.com

Currently, as a workaround, TEST_ID needs to be injected in requests, i.e.:

variables:
  username: foo
  domain: bar.com
requests:
  ...
    body:
      json:
        email: ${username}-${TEST_ID}@${domain}