GitLab SSH auth is failing with unauthorized erros

Discovered in latest omnibus packages. When attempting to take a git action on ssh we always get a 401. (git clone, push, pull, etc)

This appears to be a regression caused by !377 (merged)

In particular: https://gitlab.com/gitlab-org/gitlab-shell/-/blob/accd5e5abd03d9f08ca72068e5dafb2df090c7e3/internal/gitlabnet/client.go#L24

Is passing the secret filepath, rather than the secret itself. I can see from the rails side when debugging that the GITLAB_SHARED_SECRET_HEADER was populated with /opt/gitlab/embedded/service/gitlab-shell/.gitlab_shell_secret rather than the actual secret.