Since PR #8099, the module sets `$SSH_AUTH_SOCK` through shells' options
for interactive shell initialization instead of
`home.sessionVariablesExtra`. The replacement was not faithful, however,
since `home.sessionVariablesExtra` is sourced also in non-interactive
shells. With this commit, the shells' profile options (where
`home.sessionVariablesExtra` is sourced) are used to set
`$SSH_AUTH_SOCK`.
Fixes#8129.
