Change the build status badge to reference github actions

[Numpsy]

...instead of appveyor, which no longer has results.

I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the SharpZipLib open source product (the "Contribution"). My Contribution is licensed under the MIT License.

[codecov]

Codecov Report

Merging #616 (3bd018c) into master (d9fb8a4) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #616      +/-   ##
==========================================
+ Coverage   73.24%   73.25%   +0.01%     
==========================================
  Files          68       68              
  Lines        8712     8712              
==========================================
+ Hits         6381     6382       +1     
+ Misses       2331     2330       -1     

Impacted Files	Coverage Δ
...Code.SharpZipLib/Zip/Compression/DeflaterEngine.cs	87.45% <0.00%> (+0.33%)	⬆️

---

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 d9fb8a4...3bd018c. Read the comment docs.