Use lowest supported PHP version for shepherd run

Having Psalm installed under 8.1 leads to installing composer
dependencies that use PHP syntax incompatible with 7.4, however Psalm
analyzes itself using 7.4 parser.

.github/workflows/shepherd.yml

@@ -8,6 +8,9 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+    - uses: shivammathur/setup-php@v2
+      with:
+        php-version: '7.4'
 
     - name: Install dependencies
       run: composer install --prefer-dist --no-progress --no-suggest