Add git clone with thinpack option to support self hosted azure devops

The program uses only the hostname to address the issue of requiring thin pack to perform a git clone operation on Azure DevOps, as shown in the code snippet below.

https://github.com/coder/envbuilder/blob/d045c1ce27a5995ac60409bd48ad8e90861b6bfd/git/git.go#L56-L78

However, since Azure DevOps supports self-hosted servers, the hostname may not necessarily be dev.azure.com. Therefore, it is desired to add a command-line option to enable this functionality.

I opened a pull request below.
#446