Adapt Allen CI to changes in servers

It looks like something has changed in the CI servers that is causing the CI to fail in these servers. This MR is adapting the Allen CI to make it work again. The following errors have popped up:

• The python -m venv .venv command is failing (eg. https://gitlab.cern.ch/lhcb/Allen/-/jobs/26144360#L33). Moving to python3 solves it, which requires some adapting from eg. subprocess calls, that don't look quite the same.
• LD_LIBRARY_PATH is unbound sometimes (eg. https://gitlab.cern.ch/lhcb/Allen/-/jobs/26154674#L88). This is now fixed by setting it with a default:

export LD_LIBRARY_PATH="${LD_LIBRARY_PATH:+LD_LIBRARY_PATH:}${PWD}"