github_packages: fix bottle manifest schema violation

[ZhongRuoyu]

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According to ^{1 2}, image manifest annotations need to be a string-string map.

This should fix the errors seen in Homebrew/homebrew-core#128517, Homebrew/homebrew-core#128558, et al.

Footnotes

1. https://github.com/opencontainers/image-spec/blob/2879913ce4bc229704bf0d67b280929007b06b94/schema/image-manifest-schema.json#L35 ↩

2. https://github.com/opencontainers/image-spec/blob/2879913ce4bc229704bf0d67b280929007b06b94/schema/defs-descriptor.json#L22 ↩