BREAK: manual.yaml: don't set any default env vars

Tristan Van Berkomrequested to merge
tristan/empty-manual-element into master

This replaces !884 (closed), to squash it's commits nicely, and workaround a gitlab issue describe in this comment: !884 (comment 113751892)

This also adds the missing reference to #718 (closed) to the commit message.

Edited by Tristan Van Berkom

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