diagnostics: set last telemetry timestamp even when node is started with telemetry disabled

[dhartunian]

Today, if a node is started with the COCKROACH_SKIP_ENABLING_DIAGNOSTIC_REPORTING flag enabled, and a free/trial license is added, the behavior is confusing. Prior to the license being added, the operator has a 7 day grace period enabled for operating "without a license". After adding the license, they get immediately throttled because their "last telemetry sent" timestamp is essentially zero (and they're beyond the telemetry grace period).

Ideally, we should set the "last telemetry sent" timestamp to the node startup time regardless whether the env var is set. This will result in a less surprising experience.

Jira issue: CRDB-44790