**Backport:** https://codeberg.org/forgejo/forgejo/pulls/8821
This is a follow-up to a13414341b.
There are two situations where the previous verification command could
fail:
* The user has an SSH key in a normal file, but no running SSH agent.
* The user uses a special SSH agent, but it's not specified via the
SSH_AUTH_SOCK variable.
To fix that, we provide two separate commands to copy-paste. One
for file-based keys and one for agent-based keys. People using
file-based keys with a path other than the standard `~/.ssh/id_ed25519`
should notice themselves what to change. People using an SSH agent
get a little hint to make sure the SSH_AUTH_SOCK variable is set.
See also:
https://codeberg.org/Codeberg/Community/issues/2066
(no test or documentation changes)
### Release notes
- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.
## Testing
* `pr=8821 ; git fetch forgejo +refs/pull/$pr/head:refs/heads/wip-pr-$pr`
* `git checkout wip-pr-$pr`
* `make TAGS='sqlite sqlite_unlock_notify' watch`
* login
* visit /user/settings/keys and add an ssh key
* click verify
* see a CLI sample is displayed

* run the command matching your setup (file-based SSH key or agent-provided one), copy paste the output and submit it
* check that the verification is successful

<!--start release-notes-assistant-->
## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Bug fixes
- [PR](https://codeberg.org/forgejo/forgejo/pulls/8821): <!--number 8821 --><!--line 0 --><!--description bWFrZSBzc2gga2V5IHZlcmlmaWNhdGlvbiBjb21tYW5kIG1vcmUgcm9idXN0-->make ssh key verification command more robust<!--description-->
<!--end release-notes-assistant-->
Co-authored-by: Remo Senekowitsch <remo@buenzli.dev>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8860
Co-authored-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>
Co-committed-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>