Allow variable substitution in `variables` section

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}
```