Hey people, how can I configure hydra to checkout a private repo through ssh? I have tried adding a ssh key to the hydra user home dir, my project config looks like this:

   name    | displayname |                description                | enabled | hidden | owner |              homepage               | declfile  | decltype |              declvalue              | enable_dynamic_run_command 
-----------+-------------+-------------------------------------------+---------+--------+-------+-------------------------------------+-----------+----------+-------------------------------------+----------------------------
 nixconfig | nixconfig   | System configuration for all my computers |       1 |      0 | admin | https://github.com/timhae/nixconfig | spec.json | git      | git@github.com:timhae/nixconfig.git | f
(1 row)

(please excuse the formatting).That unfortunately gives me the following error:

May 18 23:22:24 server hydra-evaluator[376537]: starting evaluation of jobset ‘nixconfig:.jobsets (jobset#1)’ (last checked 300 s ago)
May 18 23:22:26 server hydra-evaluator[377356]: (nixconfig:.jobsets) Fetching input `decl` (git) git@github.com:timhae/nixconfig.git
May 18 23:22:28 server hydra-evaluator[377365]: fatal: ambiguous argument 'master': unknown revision or path not in the working tree.
May 18 23:22:28 server hydra-evaluator[377365]: Use '--' to separate paths from revisions, like this:
May 18 23:22:28 server hydra-evaluator[377365]: 'git <command> [<revision>...] -- [<file>...]'
May 18 23:22:28 server hydra-evaluator[377356]: command `git rev-parse master' failed with exit status 32768 in /var/lib/hydra/scm/git/ccf70cf8803f9584da653030eb51f8fc7f407e5ff30b7d976dc4ba3a1e99afe4 at /nix/store/qf86r75y8pf8y7mqv87vp9jwvq5qpy5y-hydra-2024-03-08/libexec/hydra/lib/Hydra/Helper/Nix.pm line 460.
May 18 23:22:28 server hydra-evaluator[376537]: evaluation of jobset ‘nixconfig:.jobsets (jobset#1)’ failed with exit code 1

which is expected as my default branch is 'main'. How do I tell hydra about that?