improve the build script

[MrElyazid]

Some improvements for the database build fix :

First off nice work! you managed to fix the build script with minimal changes. I added some improvements for the work done by :

• invoking the python interpreter using python3 and not python which is the standard on lots of *nix machines especially Debian based
• one sort command had sort -S 100% causing the script to exit silently due to the os killing it for consuming lots of resources, changed it to 80% ( tho in my case when i built the database locally i used -S 4G for the sort commands )
• the combine_grouped_links_file.py was loading the whole text files in a dictionary in memory, i changed that by streaming the text and sorting from the two text files instead, this helps a lot for RAM consumption ( the build process is still fast )
• I think u forgot a True keyword in line 40 in prune_pages_file.py, i removed it and corrected the condition ( i generated the database with and without this fix to test and the pages table had a little less rows after correcting the condition )