Do not push artifact that have been downloaded from the cache

From example, in this build: https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/51193892 :

[--:--:--][af6f6965][ pull:dependencies/base-sdk-image.bst] INFO    Downloaded artifact af6f6965
[00:01:05][af6f6965][ pull:dependencies/base-sdk-image.bst] SUCCESS base-sdk-bootstrap/dependencies-base-sdk-image/af6f6965-pull.147.log
[--:--:--][af6f6965][ push:dependencies/base-sdk-image.bst] START   base-sdk-bootstrap/dependencies-base-sdk-image/af6f6965-push.373.log
[--:--:--][af6f6965][ push:dependencies/base-sdk-image.bst] START   Pushing Artifact
[--:--:--][af6f6965][ push:dependencies/base-sdk-image.bst] START   Preparing compressed archive
[00:00:39][af6f6965][ push:dependencies/base-sdk-image.bst] SUCCESS Preparing compressed archive
[--:--:--][af6f6965][ push:dependencies/base-sdk-image.bst] START   Sending artifact
[00:00:54][af6f6965][ push:dependencies/base-sdk-image.bst] SUCCESS Sending artifact
[00:01:37][af6f6965][ push:dependencies/base-sdk-image.bst] SUCCESS Pushing Artifact