fix[tool]: roll back OS used to build binaries

[charles-cooper]

What I did

fix over-aggressive upgrade from #4473

How I did it

How to verify it

Commit message

per ae6ab0d3dc58167cc6, the OS image used to build vyper binaries was
updated. however, this can break users who don't have a compatible libc
(i.e., at least as recent as the latest ubuntu). roll back to an older
ubuntu for those users.

Description for the changelog

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.06%. Comparing base (dd5a3d9) to head (6bd8ebd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4494   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         120      120           
  Lines       17335    17335           
  Branches     2935     2935           
=======================================
  Hits        15960    15960           
  Misses        957      957           
  Partials      418      418           

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