Add package buildtimestamp and luet upgrade --sync

Annotate the package build time when compiling, and use that from the
client to force upgrade of packages that changed the artifact, but
didn't changed any version.

The client can trigger this behavior with `luet upgrade --sync`

pkg/compiler/compiler.go

@@ -23,6 +23,7 @@ import (
 	"regexp"
 	"strings"
 	"sync"
+	"time"
 
 	"github.com/mudler/luet/pkg/helpers"
 	. "github.com/mudler/luet/pkg/logger"
@@ -455,6 +456,8 @@ func (cs *LuetCompiler) compileWithImage(image, buildertaggedImage, packageImage
 
 	artifact.SetFiles(filelist)
 
+	artifact.GetCompileSpec().GetPackage().SetBuildTimestamp(time.Now().String())
+
 	err = artifact.WriteYaml(p.GetOutputPath())
 	if err != nil {
 		return artifact, errors.Wrap(err, "Failed while writing metadata file")