Make sure quotes in json are escaped

I'm seeing issues when I run `eas build:list --json` because git commit messages are included in the result but git commit messages sometimes contain quotes. These aren't escaped and so I am seeing invalid json in the output that looks like this:

```
{
\\...
"gitCommitMessage": "Changed some "stuff""
}
```

I've changed this so that string values (only) are escaped using `JSON.stringify`.

Author: iainbeeston

packages/eas-cli/src/utils/json.ts

@@ -33,6 +33,8 @@ function sanitizeValue(value: any): unknown {
       }
     });
     return result;
+  } else if (value && typeof value === 'string') {
+    return JSON.stringify(value)
   } else {
     return value;
   }