flex: use persistent SRCURL

[wwwutz]

preventing github getting drowned in amazon

[BEE] fetching https://github.com/westes/flex/releases/download/v2.6.4/flex-2.6.4.tar.gz
--2020-09-15 19:08:30--  https://github.com/westes/flex/releases/download/v2.6.4/flex-2.6.4.tar.gz
Location: https://github-production-release-asset-2e65be.s3.amazonaws.com/14521001/23794388-327c-11e7-8219-c41432b94acf?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20200915%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20200915T170830Z&X-Amz-Expires=300&X-Amz-Signature=aefed42a6b555e6063cc316d235473be654bb102689df4bb9daf2374967b2b2b&X-Amz-SignedHeaders=host&actor_id=0&key_id=0&repo_id=14521001&response-content-disposition=attachment%3B%20filename%3Dflex-2.6.4.tar.gz&response-content-type=application%2Foctet-stream [following]
[...]
   700K .......... .......... .......... .......... .......... 54% 8.80K 76s
   750K .......... .......... .......... .......... .......... 57% 1.88K 85s
   800K .......... .....                                       58% 68.8K=1m56s

2020-09-15 19:11:30 (7.03 KB/s) - Read error at byte 835148/1419096 (Success.). Giving up.

bee update flex-2.6.4-1