Resolve "Writer: set-env-var"

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Summary

Adding writer command set-env-var.

Description

This merge request adds the writer command set-env-var used to set an environment variable.

Acceptance criteria

• Branch has no conflicts with master

• Builds are passing

• Meaningful tests have been added and are passing

• Conform to the style guide

• Conform to the software architecture

• Documentation added

• CHANGELOG entry added

• AUTHORS entry added (if not present yet)

Issues

Closes #9 (closed)