isolate .libPaths() down function calls

[maxheld83]

this would extend #207 and #211, where currently we can toggle between different library paths within the same script, but not within the same call, where say, level 1 of the stack trace uses foo from R_LIBS_WORKFLOW and level 2 and down uses R_LIBS_ACTION or something similar.

Not even remotely aware whether this is possible, but it would be neat.

The problem arises when the "worker" function (say, desc as a dep of covr is also called in a different version by foo, while foo is run as part of covr).

Very deep backlog.