Fix build on macOS after upgrade to tezos-rust-libs 1.4

Context

Building on macOS is broken with the migration of tezos-rust-libs from 1.3 to 1.4.

After a lot of debugging and many attempts, @dannywillems noticed that adding -ccopt "-framework Security" at link time resolves the problem.

This MR does this by:

1. modifying the root dune file to add a new target:

(rule
 (target macos-link-flags.sexp)
 (action (with-stdout-to %{target}
  (system "[ '%{system}' = 'macosx' ] && echo '(-ccopt \"-framework Security\")' || echo '()'"))
 )
)

2. modifying manifest/manifest.ml to systematically include a line (:include %{workspace_root}/macos-link-flags.sexp) in link_flags section.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
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