How to specify the default value with colon for localEnv.

Hi DevContainers Team,

The case for me is to set the index url for pip, so what I do is something like

```json
{
...
"remoteEnv": {
"PIP_INDEX_URL": "${localEnv:PIP_INDEX_URL:https://pypi.org/simple}"
}
}
```

I perfer a default value to handle the cases that `PIP_INDEX_URL` does not exist. But it turns out that when the env does not exist, the corresponding env in the vscode context will be set to `https` rather than `https://pypi.org/simple`. Any clues to make it happen?

Seems it is related to the code here:
https://github.com/devcontainers/cli/blob/f7d4c853bf8c284d784173f3e915a34d961b0b55/src/spec-common/variableSubstitution.ts#L85
If I understand correctly, one option could be setting the split limit to 1, and maybe another approach is that we can avoid set the env in the context if the value is empty.

Thanks in advance.