Explicitly use Ubuntu 18.04 in CI

[elshize]

ubuntu-latest is now 20.4, and our CI setup fails on that.

[codecov]

Codecov Report

Merging #457 (6bb07b3) into master (196a6e6) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #457   +/-   ##
=======================================
  Coverage   93.21%   93.21%           
=======================================
  Files          92       92           
  Lines        4995     4997    +2     
=======================================
+ Hits         4656     4658    +2     
  Misses        339      339           

Impacted Files	Coverage Δ
include/pisa/reorder_docids.hpp	89.65% <0.00%> (+0.36%)	⬆️

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[JMMackenzie]

LGTM, but why are we failing on latest and should we be looking to fix it longer term? Thanks for taking care of this!

[elshize]

There are a few reasons it was failing. First, we use 18.04-specific LLVM repos, but that could be easily changed. The second reason is that the default gcc and clang versions are different, so some changes into installing/removing toolchains need to be done.

I figured we shouldn't be using latest anyway, because this will happen in another two years again, when they upgrade to 22.04. We should probably upgrade to 20.04 at some point, but there is no rush.