CI: Fix "nightly" job: flake8 has been replaced with ruff

Author: amotl

.github/workflows/nightly.yml

@@ -48,7 +48,7 @@ jobs:
           echo "Invoking tests with CrateDB ${CRATEDB_VERSION}"
 
           # Run linter.
-          flake8 src bin
+          poe lint
           
           # Run tests.
           bin/test -vvv