LU-16228 utils: improve lljobstats parsing

LU-16228 utils: improve lljobstats parsing

Change the hard-coded python3 interpreter path to use "env".

Allow lljobstats to parse JobIDs from Insight that have a leading
"@" if the jobname is not specified.  Otherwise it is not valid
YAML and causes the parser to choke.

Test-Parameters: trivial
Signed-off-by: Bjoern Olausson <bolausson@ddn.com>
Change-Id: Ibd7eba46fe80d39993810c06a3ada0f06ee694c6
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51964
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>