Make the variable an ArrayList to enable usage of Add() method (#14748)

build.psm1

@@ -591,7 +591,9 @@ Fix steps:
             # Special case for DSC code in PS;
             # this experimental feature requires new DSC module that is not inbox,
             # so we don't want default DSC use case be broken
-            $expFeatures = Get-ExperimentalFeature | Where-Object Name -NE PS7DscSupport | ForEach-Object -MemberName Name
+            [System.Collections.ArrayList] $expFeatures = Get-ExperimentalFeature | Where-Object Name -NE PS7DscSupport | ForEach-Object -MemberName Name
+
+            $expFeatures | Out-String | Write-Verbose -Verbose
 
             # Make sure ExperimentalFeatures from modules in PSHome are added
             # https://github.com/PowerShell/PowerShell/issues/10550