Fix running 'echo ::add-path::nightly-venv/bin'

Error: Unable to process command '::add-path::nightly-venv/bin' successfully.
214
Error: The `add-path` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

.github/workflows/testing.yml

@@ -92,6 +92,8 @@ jobs:
 
   python-nightly:
     runs-on: ubuntu-18.04
+    env:
+      ACTIONS_ALLOW_UNSECURE_COMMANDS: true
     steps:
     - uses: actions/checkout@v1
     - name: Install Python from ppa:deadsnakes/nightly