ci: test if ubuntu 18.04 has tests working

[JoanFM]

No description provided.

[github-actions]

This PR closes: #18.04

[codecov]

Codecov Report

Merging #1981 (d64aec2) into master (0f2dc89) will increase coverage by 1.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1981      +/-   ##
==========================================
+ Coverage   81.51%   82.81%   +1.30%     
==========================================
  Files         148      148              
  Lines        7107     7072      -35     
==========================================
+ Hits         5793     5857      +64     
+ Misses       1314     1215      -99     

Impacted Files	Coverage Δ
jina/enums.py	95.83% <ø> (-0.12%)	⬇️
jina/flow/base.py	80.31% <100.00%> (-0.16%)	⬇️
jina/flow/builder.py	95.52% <100.00%> (+21.60%)	⬆️
jina/parsers/flow.py	100.00% <100.00%> (ø)
jina/drivers/evaluate.py	97.14% <0.00%> (+2.85%)	⬆️
jina/drivers/encode.py	94.20% <0.00%> (+2.89%)	⬆️
jina/drivers/segment.py	100.00% <0.00%> (+5.00%)	⬆️
jina/types/sets/document.py	96.92% <0.00%> (+7.69%)	⬆️
jina/executors/indexers/cache.py	96.92% <0.00%> (+7.69%)	⬆️
jina/drivers/querylang/sort.py	100.00% <0.00%> (+8.33%)	⬆️
... and 5 more

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[github-actions]

Latency summary

Current PR yields:

• 😶 index QPS at 1295, delta to last 3 avg.: +2%
• 🐢🐢 query QPS at 20, delta to last 3 avg.: -8%

Breakdown

Version	Index QPS	Query QPS
current	1295	20
1.0.1	1275	20
1.0.0	1246	22

Backed by latency-tracking. Further commits will update this comment.