PWG_common_production_run_json: Use POSIX complient builtin for python finding
requested to merge p.-a.loizeau/cbmroot:3638_fix_python_detect_PWG_common_production_run_json into master
Reasons: which command is different in Debian and in CentOS/RockyLinux (alias + extra options)
=> This lead to trouble when vae23/vae24 started with scrontab from bare-metal virgo3 (e.g. for Weekly tests)
=> Same if container/image started by hand with apptainer from bare-metal virgo3 (e.g. for dev container testing before custom mass jobs with SLURM)
=> command -v is the advised standard (especially on bash)
=> See also https://unix.stackexchange.com/a/85250
Redmine: Refs #3638