Update nightly.yml

Downgrading python version in nightly to use same version as local

.github/workflows/nightly.yml

@@ -93,7 +93,7 @@ jobs:
       - uses: actions/checkout@v1
       - uses: actions/setup-python@v2
         with:
-          python-version: '3.x'
+          python-version: '3.8.x'
 
       - name: Install B2 CLI
         shell: bash