slurm: put slurm_install_prefix clients on the slurmd PATH (fixes Pyxis with a custom prefix)

[dholt]

Summary

slurmd runs with systemd's minimal PATH, so when slurm_install_prefix is not /usr/local the Slurm client binaries are not resolvable from slurmd's children. Enroot's 50-slurm-pmi.sh hook (installed by the pyxis role) calls scontrol and fails, and every Pyxis container refuses to start:

pyxis: container start failed with error code: 1
    [ERROR] Command not found: scontrol
    [ERROR] /etc/enroot/hooks.d/50-slurm-pmi.sh exited with return code 1

This adds a slurmd.service.d/10-slurm-path.conf drop-in that prepends {{ slurm_install_prefix }}/bin and /sbin to slurmd's PATH. The existing unconditional start slurmd task (restart + daemon-reload) in compute.yml runs after this, so the override is active on every convergence without a new handler.

The same correction was part of the closed consolidation PR #1396; this is the minimal standalone version.

Validation

• Reproduced on a fresh Ubuntu 24.04 single-GPU deployment with slurm_install_prefix=/opt/deepops/slurm, Slurm 26.05.3, Pyxis 0.11.1: srun --gpus=1 works, srun --container-image=... fails exactly as above.
• Role lint (ansible-lint, production profile): 0 failures.
• Syntax check across all release playbooks: pass.
• git diff --check: clean.
• Live re-validation with the fix is scheduled as part of the release QA rerun; results will be added here.

Notes

On Ubuntu 23.10+ Pyxis additionally needs the unprivileged user-namespace AppArmor exception for enroot-nsenter, which #1392 addresses. This PR is independent of that change.