build(release): fold the plugin manifests into the version check

herdi-win was at 0.7.3 and herdi-mac at 0.7.5, so the Windows client could not
be tagged without every install updating in a loop. All seven numbers now say
0.7.5.

The two plugin manifests move under check-versions.py rather than staying with
the `sed` in herdi-mac/build.sh, for two reasons. `sed -i ''` is the BSD
spelling: on Linux it reads the empty string as the script and the next
argument as a file, so the line only ever worked on a mac, and `|| true` made
that silent. And a mac build is the wrong trigger for a number that a Linux
relay and a Windows tray also ship -- these manifests sat at 0.5.0 through four
releases precisely because nothing that ran on those platforms wrote them.

check-versions.py is now the only writer, and the release workflow reads it.