fetch-and-ingest-branch: Send Slack notications to test channel

This change was motivated by the unintentional bug introduced in
#222 that would only be triggered
by using Slack notifications. This allows to test branches and send
notifications to the testing channel.

As part of this change, I've added an organization level variable
`TEST_SLACK_CHANNEL` that points our #scratch channel for testing
Slack notifications.

.github/workflows/fetch-and-ingest-branch.yaml

@@ -34,6 +34,8 @@ jobs:
       runtime: aws-batch
       env: |
         NEXTSTRAIN_DOCKER_IMAGE: ${{ inputs.image }}
+        GITHUB_RUN_ID: ${{ github.run_id }}
+        SLACK_CHANNELS: ${{ vars.TEST_SLACK_CHANNEL }}
         UPLOAD_CONFIG: ${{ needs.set_config_overrides.outputs.upload_config }}
       run: |
         nextstrain build \
@@ -44,6 +46,9 @@ jobs:
           --memory 64gib \
           --env AWS_ACCESS_KEY_ID \
           --env AWS_SECRET_ACCESS_KEY \
+          --env GITHUB_RUN_ID \
+          --env SLACK_TOKEN \
+          --env SLACK_CHANNELS \
           ingest \
             --configfiles config/config.yaml config/optional.yaml \
-            --config trigger_rebuild=False send_slack_notifications=False upload="$UPLOAD_CONFIG"
+            --config trigger_rebuild=False send_slack_notifications=True upload="$UPLOAD_CONFIG"