sshagent + git checkout doesn't seem to work: the `SSH_AUTH_SOCK` env var disappears from the git process environment, and thus the ssh-agent is not available to ssh started by git...

Scenario:

• create a ssh key credentials `ssh-key`
• create a pipeline job with Jenkinsfile:

  node {
      stage('Preparation') {
          sshagent (['my-ssh-key']) {
              sh 'printenv'
              sh 'ssh-add -l'
              git 'git@github.com:foo/bar.git'
          }
      }
  }
  

• run created job

Expected result:
ssh started by git finds the correct ssh key via the ssh agent.

Actual Result:

• git fetch fails : no valid ssh key found
• `ssh-add -l` works though, and shows the key added by `sshagent`

Other tests:

• `checkout` scm instead of `git` command: same behavior: it fails to find the key
• direct `sh 'ssh -vvv git@github.com'` works

More digging:

• configure a git wrapper as `git` tool in global jenkins config to observe what happens:

  #!/bin/bash                                                                                                           
  printenv >& /tmp/$$.git-env
  ssh-add -l >& /tmp/$$.git-ssh-add-list
  export GIT_TRACE=1
  export GIT_SSH_COMMAND="ssh -vvv" 
  git "${@}"
  

• run job again

Result:

• `ssh-add -l` fails: "Could not open a connection to your authentication agent."
• `printenv` shows `SSH_AUTH_SOCK` env var is not here, which explains why the ssh agent is not usable/used

So the remaining question is: why is the `SSH_AUTH_SOCK` env var removed when executing git commands ?