Disable Windows Defender entirely in CI runners

Windows Defender is a waste of time and resources on CI runners.
Disable it completely at the start of the workflow to prevent file
locking issues and speed up builds.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: WebKit ARM64 Builder

.github/workflows/build.yml

@@ -254,6 +254,18 @@ jobs:
     runs-on: ${{ matrix.runner }}
     timeout-minutes: 90
     steps:
+      - name: Disable Windows Defender
+        run: |
+          Set-MpPreference -DisableRealtimeMonitoring $true -ErrorAction SilentlyContinue
+          Set-MpPreference -DisableBehaviorMonitoring $true -ErrorAction SilentlyContinue
+          Set-MpPreference -DisableBlockAtFirstSeen $true -ErrorAction SilentlyContinue
+          Set-MpPreference -DisableIOAVProtection $true -ErrorAction SilentlyContinue
+          Set-MpPreference -DisablePrivacyMode $true -ErrorAction SilentlyContinue
+          Set-MpPreference -DisableScriptScanning $true -ErrorAction SilentlyContinue
+          Set-MpPreference -DisableArchiveScanning $true -ErrorAction SilentlyContinue
+          Set-MpPreference -SignatureDisableUpdateOnStartupWithoutEngine $true -ErrorAction SilentlyContinue
+          Set-MpPreference -DisableIntrusionPreventionSystem $true -ErrorAction SilentlyContinue
+          
       - name: Install dependencies for Windows
         run: |
           # Detect architecture