No way to specify clone depth when build context is a git repo

[ashishmax31]

Since we only care about the current state of the repo, it doesn't make sense to pull the full commit history of a repo. It would be helpful to expose a new flag to set the git clone depth - Which would then be honoured here: https://github.com/GoogleContainerTools/kaniko/blob/main/pkg/buildcontext/git.go#L62.

I'd be happy to open a PR to add this change. Thanks!