github_packages: fix bottle manifest schema violation

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.

[^1]: 2879913ce4/schema/image-manifest-schema.json (L35)
[^2]: 2879913ce4/schema/defs-descriptor.json (L22)

Library/Homebrew/github_packages.rb

@@ -363,7 +363,7 @@ class GitHubPackages
         "sh.brew.bottle.cpu.variant"        => cpu_variant,
         "sh.brew.bottle.digest"             => tar_gz_sha256,
         "sh.brew.bottle.glibc.version"      => glibc_version,
-        "sh.brew.bottle.size"               => local_file_size,
+        "sh.brew.bottle.size"               => local_file_size.to_s,
         "sh.brew.tab"                       => tab.to_json,
       }.reject { |_, v| v.blank? }