Fix example shim to actually use its task service

In commit 4b35c3829d, example shim erroneously started to depend on runc, fix that back.

Also, build example shim on all supported platforms to prevent such situations in the future.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>

runtime/v2/example/cmd/main.go

@@ -1,5 +1,3 @@
-//go:build linux
-
 /*
    Copyright The containerd Authors.
 
@@ -21,12 +19,11 @@ package main
 import (
 	"context"
 
-	_ "github.com/containerd/containerd/runtime/v2/example"
-	"github.com/containerd/containerd/runtime/v2/runc/manager"
+	"github.com/containerd/containerd/runtime/v2/example"
 	"github.com/containerd/containerd/runtime/v2/shim"
 )
 
 func main() {
 	// init and execute the shim
-	shim.Run(context.Background(), manager.NewShimManager("io.containerd.example.v1"))
+	shim.Run(context.Background(), example.NewManager("io.containerd.example.v1"))
 }