Configure Scoop to use external 7zip on ARM64

The 7zr.exe bootstrap extractor has access denied issues on ARM64.
Configure Scoop to use external 7zip mode on ARM64 to work around
this known issue.

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

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

Author: WebKit ARM64 Builder

.github/workflows/build.yml

@@ -294,7 +294,12 @@ jobs:
             echo "$env:USERPROFILE\scoop\shims" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
           }
           
-          # Install 7zip first to avoid bootstrap extraction issues
+          # Configure Scoop to use external 7zip for ARM64 to avoid extraction issues
+          if ($isARM64) {
+            scoop config use_external_7zip true
+          }
+          
+          # Install 7zip
           scoop install 7zip
           
           # Install aria2 for parallel downloads