Due to the way sshd works, gitlab-shell could be called with a single
string in the form:

```
/path/to/gitlab-shell -c key-id
```

However, due to the tightening of the regular expressions in fcff692b
this string no longer matches, so logins would fail with:

```
Failed to get username: who='' is invalid
```

This can be reproduced by changing the user's shell to point to
gitlab-shell. For example:

```
usermod git -s /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell
```

While setting gitlab-shell as the user's shell isn't officially
supported, gitlab-shell still should be able to cope with the key being
specified as the last argument. We now split the argument list and use
the last value.

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/530