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

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

## Variables

```json
{
  "input": "{{foo}}"
}
```

## Response

```json
{
  "data": {
    "hello": ""
  }
}
```

## Expected response

```json
{
  "data": {
    "hello": "{{foo}}"
  }
}
```