Script to build software stack to be used by EESSI bot

[trz42]

A script bot-build.sh could handle all environment settings etc and would thereby make the build script of the EESSI bot (eessi-bot-build.slurm) simpler.

See original post by @boegel in EESSI/eessi-bot-software-layer#10 (comment)

[trz42]

@boegel I think this can be closed as well (#233 should fix it)?

[boegel]

Agreed, closing!