Portability improvements in scripts

[vasi]

• Not all systems have GNU sed, so don't assume sed -i works
• Not all systems have /bin/bash, so use env to find bash

Tested with Calibre 7.8 on FreeBSD 14.0

Fixes #72

[Leseratte10]

Thanks for that bugfix, that looks like it could be the issue MacOS users were running into when packaging the plugin themselves.