fix: use env var to track telemetryServerUrl

[ssbushi]

• A bug causes the telemetry server started by 'genkit start' to be overridden by the ephemeral telemetry server of CLI commands like 'genkit flow:run'.
• This results in the runtime being stuck with the CLI's telemetry server URL, causing subsequent writes to fail.
• Solution in this PR is to use the GENKIT_TELEMETRY_SERVER env var to track telemetryServerUrl

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually)