sysutils/slurm-wlm: Fix: TaskPlugin=task/pgid in slurm.conf fails on FreeBSD – plugin unavailable

When setting TaskPlugin=task/pgid in slurm.conf, slurmd fails to start
and reports that the task/pgid plugin is not available in the plugins
directory.

As far as I know, pgid is an important task plugin in Slurm, used to
manage process groups. Its absence limits Slurm’s ability to manage
tasks correctly on FreeBSD.

Upstream bug report: https://support.schedmd.com/show_bug.cgi?id=23659

PR: 288668
Differential Revision: https://reviews.freebsd.org/D52143
MFH: 2025Q3

(cherry picked from commit ecdb84b12f0420490ba5ea680e2a8c8b78dc4e06)