chore: Move cmd/frontend/envvar to internal/envvar
Created by: efritz
Move from cmd/frontend to internal as it's shared code that's unspecific to the frontend.
$ ./depgraph lint NoReachingIntoCommands
(NoReachingIntoCommands) cmd/frontend/envvar: imported past command boundary by 9 packages:
- cmd/repo-updater/shared
- enterprise/internal/batches/resolvers
- enterprise/internal/insights/background
- internal/conf
- internal/database
- internal/hubspot/hubspotutil
- internal/profiler
- internal/repos
- internal/usagestats