roachtest: fix perturbation/restart/* return from setup

[andrewbaptist]

Previously the setup code for the restart tests was constructing the variation but not returning the newly constructed value. This change correctly returns the update value with the right settings. This bug was introduced due to a merge skew from previous refactorings.

Epic: none

Release note: None

[cockroach-teamcity]

This change is 

[andrewbaptist]

TFTR!

bors r=kvoli

[craig[bot]]

Build succeeded:

• acceptance
• check_generated_code
• macos_arm64_build
• docker_image_amd64
• local_roachtest
• unit_tests
• lint
• local_roachtest_fips
• linux_amd64_fips_build
• linux_arm64_build
• windows_build
• examples_orms
• linux_amd64_build
• macos_amd64_build