feat: Take dependencies into account when sorting environment variables

[siegfriedweber]

Description

Take dependencies into account when sorting environment variables

For example, the environment variables

ENV1=reference to $(ENV2)
ENV2=some value
ENV3=some value

are sorted in this order when iterated or converted to a vector:

ENV2
ENV1
ENV3

Part of stackabletech/issues#866

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added

[NickLarsenNZ]

LGTM, but left a comment to check my understanding.

[Techassi]

LGTM, just a minor (2-part) suggestion.