Deployment args formatting fix

[hobbseltoff]

kubectl.cmd is not guaranteed to return type bytes, it can also return a str so only call decode() if the type is bytes. This is causing intermittent CI failures.

Backports Required

• [ ] none - not a bug fix
• [ ] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [x] none - papercut/not impactful enough to backport
• [ ] v24.2.x
• [ ] v24.1.x
• [ ] v23.3.x

Release Notes

• None

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[vbotbuildovich]

new failures in https://buildkite.com/redpanda/redpanda/builds/55163#01922cc9-90e9-452d-b21a-24475c2eb106:

"rptest.tests.cluster_config_test.ClusterConfigTest.test_valid_settings"

new failures in https://buildkite.com/redpanda/redpanda/builds/55163#01922cc9-90e2-4eca-965b-55685724aa9e:

"rptest.tests.audit_log_test.AuditLogTestsAppLifecycle.test_recovery_mode"

new failures in https://buildkite.com/redpanda/redpanda/builds/55163#01923574-6f03-4d30-9a87-397c2c90ad84:

"rptest.tests.e2e_shadow_indexing_test.ShadowIndexingManyPartitionsTest.test_many_partitions_recovery"

[vbotbuildovich]

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/55163#01922cc9-90e2-4eca-965b-55685724aa9e

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/55163#01922ccb-514a-4391-9adb-82e25b9bc3ec

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/55163#01922f43-8816-4f5f-8f49-32761b4629a4