Shards binary not reachable on MacOS

[ysbaddaden]

We put Shards into bin/embedded on macOS: https://github.com/crystal-lang/distribution-scripts/blob/master/omnibus/config/software/shards.rb#L120

The pkg's postinst script links bin/crystal and embedded/bin/shards to the /usr/local folder: https://github.com/crystal-lang/distribution-scripts/blob/master/omnibus/package-scripts/crystal/postinst#L20-L21

But the tarball only has the crystal shell script in the bin folder. We should either put shards there directly, or at least link bin/shards to bin/embedded/shards (no need for shell script).

crystal-lang/crystal#16746