fix: install singularity on github actions

.github/workflows/build.yml

@@ -26,7 +26,8 @@ jobs:
           cache: "pip"
       - name: Install nextflow
         uses: nf-core/setup-nextflow@v1
-      - name: Install dependencies
+      - uses: eWaterCycle/setup-singularity@v7
+      - name: Install champagne
         run: |
           python -m pip install --upgrade pip setuptools
           pip install .[dev,test]