build: add sha256 checksum file for published binaries

[fguillot]

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• [X] I have tested my changes
• [X] There is no breaking changes
• [X] I really tested my changes and there is no regression
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• [X] I read this document: https://miniflux.app/faq.html#pull-request

[drpixie]

I see the checksums - many thanks :)