add codecov badge for sealer

[allencloud]

Signed-off-by: Allen Sun [email protected]

Describe what this PR does / why we need it

add codecov badge for sealer. And we see the unit test coverage of sealer is only 11%.
sealer community needs more efforts on test coverage.

Does this pull request fix one issue?

none

Describe how you did it

none

Describe how to verify it

none

Special notes for reviews

none

[codecov-commenter]

Codecov Report

Merging #1586 (a4a94bb) into main (08df4e0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1586   +/-   ##
=======================================
  Coverage   11.19%   11.19%           
=======================================
  Files          88       88           
  Lines        8067     8067           
=======================================
  Hits          903      903           
  Misses       7052     7052           
  Partials      112      112           

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