GH Actions: fix pin for setup-php

This should have the last released version as the version number, not "master".

Author: jrfnl

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
           persist-credentials: false
 
       - name: Install PHP
-        uses: shivammathur/setup-php@ec406be512d7077f68eed36e63f4d91bc006edc4 # master
+        uses: shivammathur/setup-php@ec406be512d7077f68eed36e63f4d91bc006edc4 # 2.35.4
         with:
           php-version: 'latest'
           coverage: none