fix err "non-constant format string in call to fmt.Fprintf"

[aagbsn]

Checklist

• I have read the contribution guidelines
• reference issue for this pull request: non-constant format string in call to fmt.Fprintf #1759
• if you changed anything related to how experiments work and you need to reflect these changes in the ooni/spec repository, please link to the related ooni/spec pull request:
• if you changed code inside an experiment, make sure you bump its version number

Description

Fixes some format strings

[DecFox]

Thanks for fixing this! This was long due and very annoying due to the warnings. I am going to go ahead and merge this diff.