Add simple E2E for Buidler

[cgewecke]

• clones sc-forks/buidler-e2e and installs/runs solidity-coverage at a candidate branch.

[codecov-io]

Codecov Report

Merging #438 into buidler-plugin will not change coverage.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           buidler-plugin     #438   +/-   ##
===============================================
  Coverage           99.21%   99.21%           
===============================================
  Files                  18       18           
  Lines                 766      766           
===============================================
  Hits                  760      760           
  Misses                  6        6

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