EESSI-extend EasyBuild using local EasyBuild config files

[joasode]

I was trying out EESSI at the workshop after the EuroHPC user days conference.

I was building software using the EasyBuild installation from EESSI-extend, and it unexpectantly picked up a buildpath from my local EasyBuild installation which had the value buildpath=/tmp/$USER, which exposed two behaviors:

1. EasyBuild does not resolve this path, but it does do it when starting an interactive shell script, leading to unexpected error handling. (I don't know if the config file should resolve the path; my local installation relied on setting an environment variable that was set loading my local EasyBuild module, thus not exposing the broken config file)
2. EasyBuild inside EESSI is not well isolated from system EasyBuild installations, and I think at least a warning should be produced when loading the EESSI-extend module and system configuration files are picked up.