Binary releases on github are not valid tar.gz files

[Lawrence Angrave]

The build action script creates uncompressed tar files not tar.gz files. However the file extension for the mac and linux files are still .tar.gz This leads to an error when a user attempts to decompress them e.g. with gunzip or tar -txvz

A fix will be sent as a PR. In brief, "add 'z' to the tar command in github action release.yml