ci: use PHP 8.1 for cs-fixer, drop PHP_CS_FIXER_IGNORE_ENV

php-cs-fixer ^3.92 requires PHP >=8.1, so running on 8.0 required
the IGNORE_ENV workaround. Using 8.1 removes the need for it and
ensures cs-fixer runs on a fully supported PHP version.

https://claude.ai/code/session_01WJEcEkXb8rGmmXPuF3nL95

Author: claude

.github/workflows/cs-fixer.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Install PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.0'
+          php-version: '8.1'
           tools: composer:v2.2
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -35,7 +35,7 @@ jobs:
         run: composer install --prefer-dist
 
       - name: Run PHP CS Fixer
-        run: PHP_CS_FIXER_IGNORE_ENV=1 ./bin/php-cs-fixer fix --diff
+        run: ./bin/php-cs-fixer fix --diff
 
       - name: Commit CS Fixer fixes
         uses: stefanzweifel/git-auto-commit-action@v5