ci: setting up singularity in a different way so that we don't get th…

…e error

Singularity with micromamba is erroring out:
ERROR  : Session directory does not exist: /home/runner/micromamba/envs/test-env/var/singularity/mnt/session
ABORT  : Retval = 255

.github/workflows/ci.yaml

@@ -36,11 +36,14 @@ jobs:
             snakemake
             mamba
             poetry
-            singularity
           init-shell: bash
           cache-environment: true
           post-cleanup: 'all'
 
+      - uses: eWaterCycle/setup-singularity@v7
+        with:
+          singularity-version: 3.6.4
+
       - name: Install
         shell: bash -el {0}
         run: |