feat: tart macOS vm's as job container (#33)

adds the tart:// protocol to platform mapping

e.g. `-P macos-14=tart://ghcr.io/cirruslabs/macos-sonoma-base:latest` if you have a mac.

`add-path` is probably broken

pkg/runner/run_context_other.go

@@ -0,0 +1,16 @@
+//go:build !darwin
+
+package runner
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/actions-oss/act-cli/pkg/common"
+)
+
+func (rc *RunContext) startTartEnvironment() common.Executor {
+	return func(_ context.Context) error {
+		return fmt.Errorf("You need macOS for tart")
+	}
+}