[k8s] Add checks for shuf dependency

[romilbhardwaj]

shuf is used for avoiding concurrent socat connections (#2628). However, some environments may not have shuf installed:

Tailing logs of job 1 on cluster 'redacted'...
/Users/redacted/.sky/port-forward-proxy-cmd.sh: line 48: shuf: command not found
usage: sleep seconds
Warning: Permanently added '127.0.0.1' (ED25519) to the list of known hosts.
Warning: Permanently added '10.42.56.141' (ED25519) to the list of known hosts.
I 01-21 17:51:18 log_lib.py:431] Start streaming logs for job 1.
INFO: Tip: use Ctrl-C to exit log streaming (task will not be killed).

We should add dependency checks for shuf just like we have for nc and socat.

[romilbhardwaj]

Also, shuf may get installed as gshuf on some macos environments. Since shuf and gshuf are fungible, we should auto-detect and use whichever is available.

[github-actions]

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[dongreenberg]

Bumping, ran into the shuf issue today (on Mac)

[romilbhardwaj]

Thanks @dongreenberg - our dependency on shuf will be removed after #3657.