Unable to clear env

[kyleect]

With the prod environment selected, clearing the environment doesn't do anything.

```%config
vars = ["test_value"]
secrets = ["super_secret_value"]

[prompts]
prompt_value = ""

[envs.test]
test_value = "test_value"

[envs.prod]
test_value = "prod_value"

[envs.local]
test_value = "local_value"
```

```%request
POST https://httpbin.org/post HTTP/1.1

{
  "env": "{{@env}}",
  "value": "{{:test_value}}",
  "prompted_value": "{{?prompt_value}}",
  "secret_value": "{{!super_secret_value}}"
}
```