Refine `update-from-template` workflow

[martonvago]

Just keeping track of some teething problems here:

• --overwrite is not a flag for copier update
• github_repo becomes a weird autogenerated string. Can be solved by saving it to .copier-answers.yml like copyright_year
• Files in the template/src/{{github_repo_snake_case}}/ folder are deleted on update in the child repo. Not exactly sure why it only happens for this folder.

[martonvago]

Done ✔️