Fix use of mamba in the CI

[jon-proximafusion]

CI using mamba is failing, perhaps time to update to a better workflow like this one https://github.com/fusion-energy/openmc-plasma-source/blob/main/.github/workflows/ci.yml

[shimwell]

I've updated one of the two ci scripts

also could consider adding python versions in the future

 jobs:
   testing:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.7", "3.11"]
 
     steps:
       - name: Checkout repository
@@ -38,6 +41,7 @@ jobs:
           activate-environment: ci-env
           miniforge-version: latest
           channels: conda-forge
+          python-version: ${{ matrix.python-version }}
 
       - name: install dependencies
         shell: bash -l {0}