Something like this should work (suggested by orip):
ssh-agent bash -c 'ssh-add /somewhere/yourkey; git clone [email protected]:user/project.git'if you prefer subshells, you could try the following (though it is more fragile):
ssh-agent $(ssh-add /somewhere/yourkey; git clone [email protected]:user/project.git)Git will invoke SSH which will find its agent by environment variable; this will, in turn, have the key loaded.
Alternatively, setting HOME may also do the trick, provided you are willing to setup a directory that contains only a .ssh directory as HOME; this may either contain an identity.pub, or a config file setting IdentityFile.