Add conda-forge channel explicitly

Author: SilkeSchomann

.github/workflows/unit_tests_nightly.yml

@@ -19,6 +19,7 @@ jobs:
         uses: conda-incubator/setup-miniconda@v3
         with:
           miniforge-version: latest
+          channels: conda-forge, mantid
           activate-environment: mslice-env
           environment-file: environment.yml
           auto-activate-base: false