iter8: `iter8 run` fails when using `--set duration=...`

iter8 run fails when setting duration.

Successful Execution

$ iter8 run --set url=https://example.com
INFO[2022-02-08 16:41:03] Created the experiment.yaml file containing the experiment spec
INFO[2022-02-08 16:41:03] starting experiment run
INFO[2022-02-08 16:41:03] task 1: gen-load-and-collect-metrics-http : started
INFO[2022-02-08 16:41:15] task 1: gen-load-and-collect-metrics-http : completed
INFO[2022-02-08 16:41:15] experiment completed successfully

Failed execution

$ iter8 run --set url=https://example.com         --set duration=10s
INFO[2022-02-08 16:41:23] Created the experiment.yaml file containing the experiment spec
INFO[2022-02-08 16:41:23] starting experiment run
INFO[2022-02-08 16:41:23] task 1: gen-load-and-collect-metrics-http : started
ERRO[2022-02-08 16:41:26] fortio failed                                 stack-trace=below ...
::Trace:: error 400 for https://example.com: "Get \"https://example.com\": context deadline exceeded (Client.Timeout exceeded while awaiting headers)"
ERRO[2022-02-08 16:41:26] failed to get results since fortio run was aborted
ERRO[2022-02-08 16:41:26] task 1: gen-load-and-collect-metrics-http : failure
ERRO[2022-02-08 16:41:26] exiting with code 1