ssh-agent: set $SSH_AUTH_SOCK in non-interactive shells

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.

(cherry picked from commit 89c9508bbe)

tests/modules/services/ssh-agent/darwin/bash-integration.nix

@@ -8,7 +8,7 @@
 
   nmt.script = ''
     assertFileContains \
-      home-files/.bashrc \
+      home-files/.profile \
       'export SSH_AUTH_SOCK=$(@getconf-system_cmds@/bin/getconf DARWIN_USER_TEMP_DIR)/ssh-agent'
   '';
 }