Curly brackets inside string are not properly parsed from variables input in sandbox

[super16]

Issue Description

Apollo server sandbox can't parse curly brackets inside string from variables input field. When requested from external client, curly brackets are parsed correctly, so I guess how the input is parsed causing the bug.

Link to Reproduction

https://github.com/super16/apollo-sandbox-bug-reproduction

Reproduction Steps

Minimal reproduction example with echo server from repo with reproduction.

query ExampleQuery($input: String) {
  hello(input: $input)
}

Variables

{
  "input": "{{foo}}"
}

Response

{
  "data": {
    "hello": ""
  }
}

Expected response

{
  "data": {
    "hello": "{{foo}}"
  }
}

[glasser]

I think this is the environment variables feature of Explorer, though I'm not sure if there's a way to turn it off (or restrict it only to defined environment variables).