Allow localEnv in feature environment variables.

Part of the feature spec is a containerEnv property (note - for features, not the devcontainer.json property). It would be nice if it was possible to use the localEnv reference here, so that people who use my feature aren't required to define the environment variable themselves.

Example devcontainer-feature.json

{
    "name": "Some Feature",
    "id": "some-feature",
    "version": "1.0.0",
    "description": "A Feature.",
    "containerEnv": {
        // Currently fails because the substitution isn't performed.
        "DISPLAY": "${localEnv:DISPLAY}",
    },
    // X-11 socket, for window control
    {
        "source": "/tmp/.X11-unix",
        "target": "/tmp/.X11-unix",
        "type": "bind"
    },
    "installsAfter": [
        "ghcr.io/devcontainers/features/common-utils",
    ]
}