branchSources {
   branchSource {
     source {
       github {
         ...
         traits {
           sshCheckoutTrait {
             credentialsId(CHECKOUT_SSH_CREDENTIALS_ID)            
          }
        }
       }
     }
   }
 }

Leads to:

Found multiple extensions which provide method sshCheckoutTrait with arguments [test_seed$_run_closure1$_closure2$_closure3$_closure4$_closure5$_closure6$_closure7@47c1791b]: [[com.cloudbees.jenkins.plugins.bitbucket.SSHCheckoutTrait, org.jenkinsci.plugins.github_branch_source.SSHCheckoutTrait]]