Create benchmark_cache dir if it doesn't exist

[dirtybit]

Create benchmark_cache directory if it doesn't exist before downloading the default packages.

[codecov]

Codecov Report

Merging #489 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
- Coverage   82.62%   82.58%   -0.04%     
==========================================
  Files          35       35              
  Lines        2630     2630              
  Branches      928      928              
==========================================
- Hits         2173     2172       -1     
- Misses        279      280       +1     
  Partials      178      178

Impacted Files	Coverage Δ
packages/babel-plugin-minify-simplify/src/index.js	80.6% <0%> (-0.13%)	⬇️

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[boopathi]

Thanks!