ci: Fix PYTHON_VER reference

Author: jdrew82

.github/workflows/ci.yml

@@ -118,7 +118,7 @@ jobs:
           cache-from: "type=gha,scope=${{ env.INVOKE_DIFFSYNC_IMAGE_NAME }}-${{ env.INVOKE_DIFFSYNC_IMAGE_VER }}-py${{ matrix.python-version }}"
           cache-to: "type=gha,scope=${{ env.INVOKE_DIFFSYNC_IMAGE_NAME }}-${{ env.INVOKE_DIFFSYNC_IMAGE_VER }}-py${{ matrix.python-version }}"
           build-args: |
-            PYTHON_VER=${{ env.PYTHON_VER }}
+            PYTHON_VER=${{ matrix.python-version }}
       - name: "Linting: Pylint"
         run: "poetry run invoke pylint"
   pytest:
@@ -155,7 +155,7 @@ jobs:
           cache-from: "type=gha,scope=${{ env.INVOKE_DIFFSYNC_IMAGE_NAME }}-${{ env.INVOKE_DIFFSYNC_IMAGE_VER }}-py${{ matrix.python-version }}"
           cache-to: "type=gha,scope=${{ env.INVOKE_DIFFSYNC_IMAGE_NAME }}-${{ env.INVOKE_DIFFSYNC_IMAGE_VER }}-py${{ matrix.python-version }}"
           build-args: |
-            PYTHON_VER=${{ env.PYTHON_VER }}
+            PYTHON_VER=${{ matrix.python-version }}
       - name: "Run Tests"
         run: "poetry run invoke pytest"
   changelog: