Pass the channel as environment variable

Author: mdotb-moz

.github/workflows/fireoxos-ios-testrail-milestone.yml

@@ -24,4 +24,5 @@ jobs:
           testrail-host: ${{ secrets.TESTRAIL_HOST }}
           testrail-username: ${{ secrets.TESTRAIL_USERNAME }}
           testrail-api-key: ${{ secrets.TESTRAIL_PASSWORD }}
-          webhook_url: "${{ secrets.SLACK_MOBILE_ALERTS_SANDBOX_CHANNEL }}"
+        env:
+          SLACK_MOBILE_ALERTS_SANDBOX_CHANNEL: ${{ secrets.SLACK_MOBILE_ALERTS_SANDBOX_CHANNEL }}