refactor(windows scripts): remove BOM and improve UTF-8 handling

windows/tools/update-windows.ps1

@@ -1,4 +1,4 @@
-<#
+<#
     LXS - Update Windows
     Description: Install pending Windows updates and upgrade every winget
                  package. Mirror of linux/tools/update-server.sh.
@@ -22,7 +22,10 @@ if ($LxsLibPath -and (Test-Path $LxsLibPath)) {
         Write-Error 'Failed to fetch lib/common.ps1'
         exit 1
     }
-    . ([scriptblock]::Create($LxsLibSource))
+    # The library ships with a UTF-8 BOM (Windows PowerShell 5.1 needs it to
+    # read the file as UTF-8); over HTTP that BOM arrives as a leading U+FEFF
+    # character, which the parser will not accept.
+    . ([scriptblock]::Create(($LxsLibSource -replace '^\uFEFF', '')))
 }
 
 $env:LXS_LOG_FILE = Join-Path (Get-LxsTempDir) 'lxs_update_windows.log'