build: missing bump

Author: RobPasMue

.github/workflows/nightly_docker_test.yml

@@ -21,7 +21,7 @@ on:
       - v*
 
 env:
-  MAIN_PYTHON_VERSION: '3.11'
+  MAIN_PYTHON_VERSION: '3.13'
   ANSRV_GEO_IMAGE_WINDOWS_TAG: ghcr.io/ansys/geometry:windows-latest-unstable
   ANSRV_GEO_IMAGE_LINUX_TAG: ghcr.io/ansys/geometry:linux-latest-unstable
   ANSRV_GEO_PORT: 710