simplify runner selection

[polyipseity]

The varaible ACTIONS_RUNNER_TYPE is defined for private repositories only, so public ones always use ubuntu-latest (and there is no point to using ubuntu-slim for public repos).

Kelvin — 17:49
only need to care if docker runs thru test server will never conflict with existing services

jobs:
  check:
    runs-on: ${{ vars.ACTIONS_RUNNER_TYPE || 'ubuntu-latest' }}

simplest way

Kelvin — 18:28
omit slim now