Add epoch configs as env variables to Benchnet2 Makefile

[jordanschalm]

This PR extracts epoch configs as env variables in BN2, allowing them to be overridden (see https://github.com/onflow/flow-go-internal/pull/7028).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.19%. Comparing base (c6a1f97) to head (2769e66).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6855      +/-   ##
==========================================
+ Coverage   41.15%   41.19%   +0.03%     
==========================================
  Files        2065     2109      +44     
  Lines      182499   185664    +3165     
==========================================
+ Hits        75116    76485    +1369     
- Misses     101077   102768    +1691     
- Partials     6306     6411     +105     

Flag	Coverage Δ
unittests	41.19% <ø> (+0.03%)	⬆️

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