Add github action to regularly check for new rust releases and add build images

[abizjak]

Task description

Cargo concordium verifiable builds require a fixed docker image as the context where the contract is built.

We publish these images for each rust version but this is currently a matter of remembering to do it.

It'd be good to add a scheduled github action that would

• each week check the latest rust version
• check if a docker image for verifiable builds already exists
• and if not make the image based on the latest rust version.