This is how I reverse engineered the Instagram app + included linux binaries to locate the private key used for…
Something like this should work (suggested by orip):
ssh-agent bash -c 'ssh-add /somewhere/yourkey; git clone email@example.com: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 firstname.lastname@example.org:user/project.git)
Git will invoke SSH which will find its agent by environment variable; this will, in turn, have the key loaded.
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.