slapos.cookbook test: use environment variables to pass eggs directories

slapos.cookbook recipe tests sometimes manipulate buildout eggs working set,
which will install packages from pypi. makeRecipe utilty function was
guessing buildout egg folders by parsing builout.cfg, but since a33844a2
(slapos-testing, slapos-sr-testing: Don't duplicate instance.cfg prologue
that is already provided by stack/nxdtest/instance.cfg, 2021-03-22),
these are defined in an extended profile, so this stopped working.

Instead of this fragile pattern, make the test software release pass the eggs
directories with an explicit environment variable.