[Fix] Fix CI by using ubuntu 18.04 in github workflow

[yezhen17]

No description provided.

[yezhen17]

Credit to open-mmlab/mmcv#883

[codecov]

Codecov Report

Merging #350 (bcd29fa) into master (b65f9b0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #350   +/-   ##
=======================================
  Coverage   49.69%   49.69%           
=======================================
  Files         174      174           
  Lines       11758    11758           
  Branches     1838     1838           
=======================================
  Hits         5843     5843           
  Misses       5555     5555           
  Partials      360      360           

Flag	Coverage Δ
unittests	49.69% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8b131f5...c6bae2e. Read the comment docs.