hooks: Correctly filter environment variables for custom hooks

This MR is the last part of hook cleanups for now. It cleans up some of our hook tests by removing needless setups and checks, moves filtering of environment variables from gitaly-hooks into the hook manager, unifies all code paths to compute custom hook environments, removes the hooks payload from custom hooks payload and finally fixes that we missed to pass GIT_DIR to them during the Ruby->Go conversion of hooks.

Fixes #3211 (closed)