`ubuntu-slim` should natively supported

[koppor]

Act version

0.2.84

Feature description

Have a workflow with ubuntu-slim image

See

Skipping unsupported platform -- Try running with `-P ubuntu-slim=...`

Fallback should be the platform used for ubuntu-latest

Workaround:

-P ubuntu-slim=ghcr.io/catthehacker/ubuntu:act-latest

(Found at #5995 (comment))