ci: mangen step doesn't substitute version in manpages

[tranzystorekk]

The newly added mangen step in releases seems to be missing the $version substitution that is done in just man, so manpage artifacts are missing the proper version string.

[Vishal-770]

I'd like to take this on — confirmed the bug: the mangen recipe in the justfile skips the sed version substitution that man does. Will submit a PR shortly.