Fix psalm version to 4.18.1 when running on GitHub

Author: dennisvdvliet

.github/workflows/psalm.yml

@@ -16,4 +16,4 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Psalm – Static Analysis for PHP
-        uses: docker://vimeo/psalm-github-actions
+        uses: docker://vimeo/psalm-github-actions:4.18.1