Make the cog commands' prefix configurable in the `.env.*`

[ctmbl]

RootPythia/src/bot/root_pythia_bot.py

Line 43 in edbb5ff

_PREFIX = "!"

This should be changed to getenv the prefix

[ctmbl]

thiss should be updated as well: https://github.com/iScsc/RootPythia/pull/30/files/555d01c6327e27f9bd054deb12b4b763600c0765#r1283561559