dev/sg: add envOverrides option for sets to override command env
Created by: bobheadxi
Right now, command env takes precedence over set env. I'm scared to change this, so instead I added envOverrides
and prop-drilled it to override whatever env is set on the command.
Test plan
Tested in https://github.com/sourcegraph/sourcegraph/pull/38319 , frontend-e2e
tests work now