Git protocol environment variable should be passed onto Gitaly

It seems that gitlab-shell supplies Gitaly with the git protocol from the access verification response, rather than simply sourcing it from the environment variable. This doesn't completely make sense since the user ultimate specifies whether protocol v2 is desired on a session-by-session basis. The environment variable GIT_PROTOCOL should be sourced to help make a determination on whether to pass the variable onto Gitaly.

Originally discovered in gitlab#207382 (closed)

Relates to gitlab#27828 (closed)