[CORE-6999] dt: deflake offset retention test

[IoannisRP]

Fixes: CORE-6999

The first rpk produce command sometimes takes a bit more time.

Add an initial call with higher timeout.

Note: A different dt test was failing this time, for the same reason, so PT added it in the existing ticket.

Backports Required

• [x] none - not a bug fix
• [ ] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v25.3.x
• [ ] v25.2.x
• [ ] v25.1.x

Release Notes

• none

[vbotbuildovich]

CI test results

test results on build#77535

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
MountUnmountIcebergTest	test_simple_remount	{"cloud_storage_type": 1}	integration	https://buildkite.com/redpanda/redpanda/builds/77535#019aff61-7612-4d62-b672-a734e0bf637b	FLAKY	8/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.2014, p0=0.6283, reject_threshold=0.0100. adj_baseline=0.4906, p1=0.0616, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=MountUnmountIcebergTest&test_method=test_simple_remount