VSCode Variable Interpolation

VSCode performs its own variable interpolation on variables set in `EnvironmentVariableCollection`. Given the following `.envrc`:

```
export WOW='${workspaceFolder}'
```

In regular terminal:

```
❯ echo $WOW
${workspaceFolder}
```


In VSCode integrated terminal:

```
❯ echo $WOW
/Users/zhaofeng/Sandbox
```

What's worse, exporting any variable containing `${env}` locks up the VSCode terminal.