update ci acceptance job flag from latest agent to nightly

Author: SugatD

.github/workflows/ci.yml

@@ -16,4 +16,4 @@ jobs:
     uses: "puppetlabs/cat-github-actions/.github/workflows/module_acceptance.yml@main"
     secrets: "inherit"
     with:
-      flags: "--latest-agent"
+      flags: "--nightly"