Add binaries for mac and linux on tag via github actions

[lutostag]

This was a bit tricky to get sorted properly. The main hurdle was figuring out about static linking with crystal https://crystal-lang.org/reference/1.3/guides/static_linking.html#macos. I have downloaded the binaries built at https://github.com/lutostag/justprep/releases/tag/v1.0.9 and they work on my two different machines.

In order to install them I just 😉

# download the appropriate binary from the latest release
chmod a+x justprep-*
mv justprep-* /usr/local/bin/justprep