Add Xcode 12.4 to the test.yml workflow

[MaxDesiatov]

No description provided.

[codecov]

Codecov Report

Merging #149 (c2d3d15) into main (f32280e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #149   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files           9        9           
  Lines         312      312           
=======================================
  Hits          297      297           
  Misses         15       15           

Impacted Files	Coverage Δ
Sources/CoreXLSX/XLSXFile.swift	92.18% <100.00%> (ø)

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