feat: allow `tag` in plugin config for default tag replacement

We previous defaulted the `tag` of an image to be `${nextRelease.version}`.
However, we may want to overide this default when doing tag replacement.

A common use case for this if the incoming tag has parts to it:

```json
tag: "${tag.split('-').slice(0,-1).join('-')}-${nextRelease.version}"
images:
  "${CI_REGISTRY_IMAGE}/node:16-${CI_COMMIT_SHA}"
  "${CI_REGISTRY_IMAGE}/node:18-${CI_COMMIT_SHA}"
```