feat(platforms): fall back to ubuntu-latest image for ubuntu-slim runner

[toller892]

Summary

GitHub Actions' ubuntu-slim 1-vCPU runner (in public preview since 2025-10-28) uses the same base distro as ubuntu-latest. Today act prints:

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

and the user has to manually pass -P ubuntu-slim=ghcr.io/catthehacker/ubuntu:act-latest every time, which defeats the point of having a fallback for "latest" Ubuntu runners.

This change adds ubuntu-slim to the default platform map in cmd/platforms.go, resolving it to the same image as ubuntu-latest (node:16-buster-slim). Users can still override it via -P ubuntu-slim=... exactly like the other platforms — newPlatforms() already supports overriding any default.

Changes

• cmd/platforms.go: add ubuntu-slim → node:16-buster-slim to the default platforms map (1 line).
• cmd/platforms_test.go (new): two unit tests covering the default and the user-override path.

Testing

$ go test ./cmd/ -run 'TestNewPlatforms' -v
=== RUN   TestNewPlatformsDefaults
--- PASS: TestNewPlatformsDefaults (0.00s)
=== RUN   TestNewPlatformsUserOverride
--- PASS: TestNewPlatformsUserOverride (0.00s)
PASS
ok      github.com/nektos/act/cmd       0.782s

go vet ./cmd/ passes. Pre-existing TestFlags/TestFlag-bug-report failure is unrelated (requires a running Docker daemon).

Closes #6003