Improve yaml variable substitution

[jcthomassie]

• Allow substitution of variables defined in build file
• Remove shellexpand
• Deserialize all strings with substitution

repo:
  local: ${{ env.HOME }}/dotfiles
...
  - ${{ repo.local }}-rest-of-string  # variables defined in file
  - ${{ env.DOTS_REPO_ROOT }}-rest-of-string  # environment variables