While there is no at-will cantrip that directly heals, the unlockable illithid power Transfuse Health lets one heal another character at the cost of half one's current hp. Now you'll be thinking, "that's not infinite", and you'd be mostly right - except warlocks can create 7 temp hp at will with the eldritch invocation Fiendish Vigor. The neat thing about transfuse health is that it does not transfer the healing character's hp to the healed one - no, it subtracts the amount of half the character's current hp and then heals the other character for that, BUT, it starts subtracting from temporary hp! This means that as long as you have 15 hp and 7 points of fiendish vigourous temp hp up, the seven hp that make up half your current actual hp will be subtracted from your temp hp and not touch your real hp, leaving you with 15 perfectly fine and untouched hp. Reapply fiendish vigour and repeat as often as necessary.

It may not be the fastest way of healing, but it is repeatable ad infinitum on other group members and only leaves the healer at 15 hp + 7 temp hp.