flatpak_repo: explicitly define a unique key

Abderrahim Kitounirequested to merge
abderrahimk/bst-external:abderrahim/flatpak-repo-cache-key into master

Since we don't set the commands at configure time, we can't rely on the ScriptElement implementation of get_unique_key.

Bump BST_ARTIFACT_VERSION accordingly

Depends on !69 (merged), please merge that first.

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