CI: fix typo in build

It seems Python 3.6 and 3.10 were actually not tested because apparently the build.yaml only tested for Python 3.8.

.github/workflows/main.yml

@@ -33,7 +33,7 @@ jobs:
     - name: Setup Python
       uses: actions/setup-python@v2
       with:
-        python-version: ${{ matrix.python }}
+        python-version: ${{ matrix.python-version }}
 
     - name: Install dependencies
       run: |