Use official Codecov action

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Turns out there's now an official CodeCov action. Giving it a shot to see if it fixes our issues.
https://github.com/marketplace/actions/codecov

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3b50f33). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1214   +/-   ##
=========================================
  Coverage          ?   82.74%           
=========================================
  Files             ?      695           
  Lines             ?    30252           
  Branches          ?     3429           
=========================================
  Hits              ?    25033           
  Misses            ?     4523           
  Partials          ?      696           

Flag	Coverage Δ
#unittests	82.74% <ø> (?)

Impacted Files	Coverage Δ
...Formats/Gif/Sections/GifGraphicControlExtension.cs	95.65% <0.00%> (ø)
...Metadata/Profiles/ICC/Various/IccPositionNumber.cs	58.33% <0.00%> (ø)
...rocessing/Extensions/Effects/PixelateExtensions.cs	100.00% <0.00%> (ø)
...c/ImageSharp/Formats/Tga/TgaImageTypeExtensions.cs	100.00% <0.00%> (ø)
src/ImageSharp/Formats/Png/Adam7.cs	78.57% <0.00%> (ø)
...ocessors/Transforms/Resamplers/BicubicResampler.cs	100.00% <0.00%> (ø)
...malization/GlobalHistogramEqualizationProcessor.cs	100.00% <0.00%> (ø)
src/ImageSharp/IO/DoubleBufferedStreamReader.cs	97.29% <0.00%> (ø)
src/ImageSharp/GeometryUtilities.cs	100.00% <0.00%> (ø)
...mageSharp/Metadata/Profiles/ICC/IccTagDataEntry.cs	46.66% <0.00%> (ø)
... and 685 more

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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