Replace i386 tests with arm64 in CI workflow

i386 support has been dropped upstream in setup-php due to deb.sury
removing i386 packages.

See: shivammathur/setup-php#1040 (comment)

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

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

Author: oschwald

.github/workflows/test.yml

@@ -14,14 +14,10 @@ jobs:
     container: shivammathur/node:latest-${{ matrix.arch }}
     strategy:
       matrix:
-        arch: ["amd64", "i386"]
+        arch: ["amd64", "arm64"]
         operating-system: [ubuntu-latest]
         php-version: ['7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4']
         php-extension: ['bcmath', 'gmp']
-        exclude:
-          # We are getting weird libxml2 failures on this combo
-          - arch: "i386"
-            php-version: 8.4
 
     name: "PHP ${{ matrix.php-version }} (with ${{ matrix.php-extension }}) test on ${{ matrix.operating-system }}/${{ matrix.arch }}"
     steps: