The link to gerbil_data.zip has to be manually updated in Github actions

[MichaelRoeder]

Problem

The github action script uses a hard-coded link to access the gerbil_data.zip file. This link has to be updated manually. However, this creates additional work and makes the action performing the download in a different way than most users.

Solution

• extend the start script with a command line option that skips the download of indexes
• make use of the start script instead of the hard-coded download in the github action