Are you certain that it is "stuck at checkout"?
When you list the processes on the machine running the build, is there a "git.exe" process running? I was able to clone that repository to my Windows machine without any trouble. I can't build that Jenkinsfile (don't have Salesforce installed, don't have a Salesforce account, don't have the tools needed for Salesforce).
What version of git are you running? The command "git --version" will report the version of git installed on that computer.
If you change the global tool configuration for the git location from 'C:\Program Files\Git\cmd\git.exe' to 'C:\Program Files\Git\bin\git.exe', does it behave any differently?
Are you using a freestyle job, a pipeline job, or a multi-branch pipeline to perform the checkout?
What is the configuration of the job which is performing the checkout?
What is the next step after the checkout completes?
Is that step stuck?
Mark Waite