Steps to reproduce -

Manage Jenkins => Configure System => Global properties => Environment variables
Add
Name = GITHUB_HOST
Value = <github IP address>

Name = GITHUB_REPO
Value = <github repo path>

Create a job
Under job configuration define git repository as below
https://$

{GITHUB_HOST}/${GITHUB_REPO}
provide user/pass for credential
complete the job configuration

Trigger SCM poll by using curl (Note - provide your Jenkins host in following command)

curl http://${JENKINS_SERVER}/jenkins/git/notifyCommit?url=[https://\$|https://%24/]"{GITHUB_HOST}"/\$"{GITHUB_REPO}"

It'll poll the corresponding job but if you see Git SCM poll log, it's dumping the following error
In short - It's not able to expand the global variable

> /usr/bin/git --version # timeout=10
> /usr/bin/git -c core.askpass=true fetch --tags --progress https://${GITHUB_HOST}

/${GITHUB_REPO} +refs/heads/:refs/remotes/origin/
FATAL: hudson.plugins.git.GitException: Failed to fetch from https://$

{GITHUB_HOST}/${GITHUB_REPO}
hudson.util.IOException2: hudson.plugins.git.GitException: Failed to fetch from https://${GITHUB_HOST}

/${GITHUB_REPO}
at hudson.plugins.git.GitSCM.compareRemoteRevisionWith(GitSCM.java:573)
at hudson.scm.SCM.compareRemoteRevisionWith(SCM.java:381)
at hudson.scm.SCM.poll(SCM.java:398)
at hudson.model.AbstractProject.pollWithWorkspace(AbstractProject.java:1468)
at hudson.model.AbstractProject._poll(AbstractProject.java:1438)
at hudson.model.AbstractProject.poll(AbstractProject.java:1349)
at jenkins.triggers.SCMTriggerItem$SCMTriggerItems$Bridge.poll(SCMTriggerItem.java:119)
at hudson.triggers.SCMTrigger$Runner.runPolling(SCMTrigger.java:526)
at hudson.triggers.SCMTrigger$Runner.run(SCMTrigger.java:555)
at hudson.util.SequentialExecutionQueue$QueueEntry.run(SequentialExecutionQueue.java:119)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)
Caused by: hudson.plugins.git.GitException: Failed to fetch from https://$

{GITHUB_HOST}/${GITHUB_REPO}
at hudson.plugins.git.GitSCM.fetchFrom(GitSCM.java:810)
at hudson.plugins.git.GitSCM.compareRemoteRevisionWithImpl(GitSCM.java:701)
at hudson.plugins.git.GitSCM.compareRemoteRevisionWith(GitSCM.java:571)
... 14 more
Caused by: hudson.plugins.git.GitException: Command "/usr/bin/git -c core.askpass=true fetch --tags --progress https://${GITHUB_HOST}

/${GITHUB_REPO} +refs/heads/:refs/remotes/origin/" returned status code 128:
stdout:
stderr: remote: Invalid username or password.