add be start directly feature

Quite a few changes

• flake's input of nur-kapack is based on https://github.com/oar-team/nur-kapack/tree/expes_cigri_quentin_be which contains the following feature:
• use oarsub -t besteffort -t assign=be_walltime:10 ... to override the walltime of a job ONLY during scheduling (will be replaced by 10 in this example: assign=be_walltime:).
• the nur-kapack branch is a rebase of expe_cigri_quentin on regale-2211 (idk if it was a good idea ...)