Add dynamic matrix to refresh lockfile job

.github/workflows/refresh-lockfiles.yml

@@ -23,19 +23,29 @@ on:
 
 jobs:
 
+  get_python_matrix:
+    # Determines which Python versions should be included in the matrix used in
+    # the gen_lockfiles job.
+    if: "github.repository == 'SciTools/iris'"
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.get_py.outputs.matrix }}
+    steps:
+      - uses: actions/checkout@v3
+      - id: get_py
+        run: echo "::set-output name=matrix::$(ls -1 requirements/ci/py??.yml | xargs -n1 basename | sed 's/....$//' | jq -cnR '[inputs]')"
+
   gen_lockfiles:
     # this is a matrix job: it splits to create new lockfiles for each
     # of the CI test python versions.
-    # this list below should be changed when covering more python versions
-    # TODO: generate this matrix automatically from the list of available py**.yml files
-    #       ref: https://tomasvotruba.com/blog/2020/11/16/how-to-make-dynamic-matrix-in-github-actions/
     if: "github.repository == 'SciTools/iris'"
     runs-on: ubuntu-latest
+    needs: get_python_matrix
 
     strategy:
       matrix:
-        python: ['38']
-    
+        python: ${{ fromJSON(needs.get_python_matrix.outputs.matrix) }}
+
     steps:
       - uses: actions/checkout@v3
       - name: install conda-lock
@@ -44,12 +54,12 @@ jobs:
           conda install -y -c conda-forge conda-lock
       - name: generate lockfile
         run: |
-          $CONDA/bin/conda-lock lock -k explicit -p linux-64 -f requirements/ci/py${{matrix.python}}.yml
-          mv conda-linux-64.lock py${{matrix.python}}-linux-64.lock
+          $CONDA/bin/conda-lock lock -k explicit -p linux-64 -f requirements/ci/${{matrix.python}}.yml
+          mv conda-linux-64.lock ${{matrix.python}}-linux-64.lock
       - name: output lockfile
         uses: actions/upload-artifact@v2
         with:
-          path: py${{matrix.python}}-linux-64.lock
+          path: ${{matrix.python}}-linux-64.lock
 
   create_pr:
     # once the matrix job has completed all the lock files will have been uploaded as artifacts.