ci: Unset all gitlab CI_* variables before running GHC
The goal here is to somewhat sanitize the environment so that
performance tests don't fluctuate as much as they have been doing. In
particular the length of the commit message was causing benchmarks to
increase because gitlab stored the whole commit message twice in
environment variables. Therefore when we used getEnvironment it would
cause more allocation because more string would be created.
See #20431