Use conda instead of mamba

Author: SilkeSchomann

.github/workflows/unit_tests.yml

@@ -25,8 +25,7 @@ jobs:
 
       - name: Install Mantid
         run: |
-          # mamba install -c mantid/label/nightly mantid mantidqt
-          mamba install -c mantid mantid mantidqt
+          conda install -c mantid/label/nightly mantid mantidqt
 
       - name: Run Tests and Coverage
         run: |