Fix version number for local Chrome releases

[klappradla]

The Github-release workflow sketched out in #249 uploads a bundled Chrome release to Github. The contents of this archive are intended for "local" installations in Chrome.

Due to Mozilla not allowing us to sign the same version as "listed" and "unlisted" in Firefox Add Ons, we appended an unlisted string to the version name. Unfortunately this makes the version name then incompatible with Chrome 🤯.

One cannot install the released version as unpacked extension in Chrome:

We need to find a way to have the custom version name either only in Firefox or change the pattern to be compatible with Chrome as well.