How to specify the private SSH-key to use when executing shell command on Git?

A rather unusual situation perhaps, but I want to specify a private SSH-key to use when executing a shell (git) command from the local computer.

Basically like this:

git clone git@github.com:TheUser/TheProject.git -key "/home/christoffer/ssh_keys/theuser"

Or even better (in Ruby):

with_key("/home/christoffer/ssh_keys/theuser") do
  sh("git clone git@github.com:TheUser/TheProject.git")
end

I have seen examples of connecting to a remote server with Net::SSH that uses a specified private key, but this is a local command. Is it possible?