The kaniko image does not seem to have a shell installed which is what’s causing that error.
It should be possible from your CircleCI config to add an explicit entrypoint: https://circleci.com/docs/configuration-reference#docker. Can you try adding entrypoint: sh in your circleci config right after the image key?
There’s a possibility though that the image has sh missing in which case it won’t work…in which case you would need to build your own image that is based off of the kaniko image. I am trying to get an example on our docs but it may take me a few days.