winpe-status-push: brace var names before colon (parser bug)

PowerShell parses $var: as scope-namespaced syntax (e.g. $env:NAME,
$global:foo). The line

    Log "server=$PxeServer:$Port  pctype=$PCType"

errored at line 26 col 13 - parser interpreted $PxeServer: as a scope
prefix and bailed. Fix: use ${PxeServer}:${Port} so the colon is
literal. The $uri line below already had the right form.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

playbook/winpe-status-push.ps1

@@ -23,7 +23,7 @@ function Log {
 }
 
 Log "=== winpe-status-push start ==="
-Log "server=$PxeServer:$Port  pctype=$PCType"
+Log "server=${PxeServer}:${Port}  pctype=$PCType"
 
 # Serial via wmic. Always available in WinPE 10+.
 $serial = ''