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