Correctly fixe how selected packages are defined (#30039)

I will skip the run of the test suite.

- It will take 3 hours to run.
- It will not test the present code path, I will do a follow-up on that.
- Teams are blocked over this in the release manager.
- This code is now exactly the same as 7.17 https://github.com/elastic/beats/blob/7.17/dev-tools/mage/crossbuild.go#L47

dev-tools/mage/crossbuild.go

@@ -44,7 +44,7 @@ const defaultCrossBuildTarget = "golangCrossBuild"
 var Platforms = BuildPlatforms.Defaults()
 
 // SelectedPackageTypes is the list of package types
-var SelectedPackageTypes []PackageType = []PackageType{}
+var SelectedPackageTypes []PackageType
 
 func init() {
 	// Allow overriding via PLATFORMS.