Use Ubuntu 20.04 and renable assert builds

[vchuravy]

No description provided.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.09 ⚠️

Comparison is base (9487eb8) 78.11% compared to head (95ab2cc) 78.03%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #883      +/-   ##
==========================================
- Coverage   78.11%   78.03%   -0.09%     
==========================================
  Files          18       18              
  Lines        8417     8417              
==========================================
- Hits         6575     6568       -7     
- Misses       1842     1849       +7     

see 2 files with indirect coverage changes

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[vchuravy]

Assert builds would need JuliaCI/julia-buildkite#303 or https://github.com/maleadt/LLVM.jl/blob/e44c430c4a48225114f217c36b166b85a3fcbe5a/.github/workflows/ci.yml#L61-L108

[vchuravy]

Assertions are green, which is nice.