add missing easystack file (2021b) to test

Author: truib

.github/workflows/test_eessi.yml

@@ -20,6 +20,7 @@ jobs:
         - x86_64/generic
         EASYSTACK_FILE:
         - eessi-2023.06-eb-4.7.2-2021a.yml
+        - eessi-2023.06-eb-4.7.2-2021b.yml
     steps:
         - name: Check out software-layer repository
           uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8 # v3.1.0