Make MCORE_LIBS prioritize earlier entries

[elegios]

This change makes the MCORE_LIBS environment variable behave a touch more like similar variables for other languages (or PATH): earlier entries are prioritized over later ones. Previously the behavior was the opposite.

This is relevant for the self-contained version of TreePPL (see treeppl/treeppl-python#9), which adds its own versions of the various libraries to MCORE_LIBS before starting. Potential options for how to get the desired behavior there include:

• Look at the MCORE_LIBS in the users environment, remove entries for stdlib, coreppl, and treeppl, then add the correct ones. This removes duplicates, but is fiddly to get right in bash.
• Add entries to the end of the environment variable. This is inconsistent with other similar variables, which feels like a potential footgun.
• Add entries to the beginning of the environment variable, and merge this PR.

I would prefer the last option (hence this PR), since it's the most consistent with the rest of the world, and it shouldn't affect any normal setups.