Jenkins pipeline git checkout access GIT_COMMIT when default checkout is skipped - jenkins

Question

Observations:
Jenkinsfile
pipeline {
agent { node 'master' }
stages {
stage('Hello') {
steps {
echo env.GIT_COMMIT
}
}
}
}
Output:
[Pipeline] }
[Pipeline] // stage
[Pipeline] withEnv
[Pipeline] {
[Pipeline] stage
[Pipeline] { (Hello)
[Pipeline] echo
e5b4b2deed1c8db486cef5641ef14c61fd48f9ed
[Pipeline] }
[Pipeline] // stage
[Pipeline] }
[Pipeline] // withEnv
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Jenkinsfile
pipeline {
agent { node 'master' }
options {
skipDefaultCheckout(true)
}
stages {
stage('Hello') {
steps {
checkout([$class: 'GitSCM', branches: [[name: '*/master']], extensions: [], userRemoteConfigs: [[url: '/tmp/r']]])
echo env.GIT_COMMIT
}
}
}
}
Output:
[Pipeline] node
Running on Jenkins in /var/jenkins_home/workspace/HelloWorld_master
[Pipeline] {
[Pipeline] stage
[Pipeline] { (Hello)
[Pipeline] checkout
Selected Git installation does not exist. Using Default
The recommended git tool is: NONE
No credentials specified
> git rev-parse --resolve-git-dir /var/jenkins_home/workspace/HelloWorld_master/.git # timeout=10
Fetching changes from the remote Git repository
> git config remote.origin.url /tmp/r # timeout=10
Fetching upstream changes from /tmp/r
> git --version # timeout=10
> git --version # 'git version 2.30.2'
> git fetch --tags --force --progress -- /tmp/r +refs/heads/*:refs/remotes/origin/* # timeout=10
> git rev-parse refs/remotes/origin/master^{commit} # timeout=10
Checking out Revision e5b4b2deed1c8db486cef5641ef14c61fd48f9ed (refs/remotes/origin/master)
> git config core.sparsecheckout # timeout=10
> git checkout -f e5b4b2deed1c8db486cef5641ef14c61fd48f9ed # timeout=10
Commit message: "init"
First time build. Skipping changelog.
[Pipeline] echo
null
[Pipeline] }
[Pipeline] // stage
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Question:
I am using jenkins version 2.349 and the project is of type Multi-branch pipeline.
As I read on the Jenkins git plugin's page, it is supposed to inject the environment variables but it seems it only does when the default checkout isn't skipped but not when we explicitly checkout. I could only find ways to access those SCM values (in my case GIT_COMMIT) by using a script { } block. Is there a way to get those variables without jumping into a script block?