Configure build-cache location when using buildah

Description

Currently there is an input to allow the override for the cache location when using kaniko (KANIKO_SNAPSHOT_IMAGE_CACHE, kaniko-snapshot-image-cache).

As we are moving from kaniko to buildah due to kaniko deprecation (#104 (closed)) I would propose to generalize the image cache location.

When using buildah the cache location is fixed to <snapshot_image_location>/cache

This is problematic in combination when using the gitlab container registry, as gitlab only allow two levels of 'folders' in its registry (see gitlab-org/gitlab#438563).

As we need the two levels already to structure the several images, appending /cache often is problematic.

I would therefore like to configure the cache location for buildah like it is already possible with kaniko

Implementation ideas

I would like to generalize the kaniko-snapshot-image-cache input. My suggestion would be build-cache-location. For kaniko and buildah it will default to <snapshot image repository> + /cache.

This however is a breaking change, as I would remove kaniko-snapshot-image-cache. Is this fine?