signature check: report to Slack (#7568)

If a tree falls in the forest and all that.

CHANGELOG_BEGIN
CHANGELOG_END

ci/bash-lib.yml

@@ -100,3 +100,4 @@ steps:
     }
     END
     echo "##vso[task.setvariable variable=${{parameters.var_name}}]$TMP"
+  displayName: install Bash lib

ci/cron/daily-compat.yml

@@ -234,3 +234,4 @@ jobs:
         displayName: check releases
         env:
           GCRED: $(GOOGLE_APPLICATION_CREDENTIALS_CONTENT)
+      - template: ../daily_tell_slack.yml