Using string buffer for faster string concatenation

[daviel]

This increases the performance of the script. For more information see https://www.lua.org/pil/11.6.html.
Would love to see string buffers in the whole script for faster development and testing. 👍

Before:

time ./update.sh 

real	0m8,036s
user	0m7,084s
sys	0m0,950s

After:

time ./update.sh 

real	0m4,633s
user	0m4,596s
sys	0m0,038s

[rm-code]

Thanks 👍