Introduced new environment config variable

[jackal1-66]

O2DPG_MC_CFG_ROOT is going to replace O2DPG_ROOT inside the config files. This will make the system more flexible by allowing the user to change this variable on-the-fly, and hence not be limited by the loaded version of O2DPG. At environment loading it will be by default equal to O2DPG_ROOT, as specified in PR 5649

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[jackal1-66]

O2DPG_MC_CFG_ROOT changed to O2DPG_MC_CONFIG_ROOT for clarity

[jackal1-66]

build/O2DPG/sim/o2 will fail until alisw/alidist#5649 is approved. The current PR must be merged only after that one.

[sawenzel]

This should fix (or be the foundation for) https://its.cern.ch/jira/browse/O2-5366

@chiarazampolli @alcaliva : I think we could merge it if you are fine with it. (Not that backports after a merge would also need the corresponding commit in alidist/o2dpg alisw/alidist#5649 ... maybe we should backport
this in any case).

update: The CI is failing. We are investigating if this is caused by the present PR... so let's wait a bit before final merge.

[jackal1-66]

Issue seems related to gSystem->ExpandPathName not being able to expand ${O2DPG_MC_CONFIG_ROOT}. It could be that the variable is not set, even though the PR on alidist has been merged, since locally I get the same error message on ROOT when I unset manually the variable.

[jackal1-66]

This merged PR #1757 manually sets the new configs variable to O2DPG_ROOT inside the test generators script. This should fix the failing CI