Avoid downloading MinGW artifact if "JULIA_CC" is set

[topolarity]

see JuliaLang/JuliaC.jl#67

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.19%. Comparing base (a9af8ed) to head (844987b).

Files with missing lines	Patch %	Lines
src/PackageCompiler.jl	86.36%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1069      +/-   ##
==========================================
- Coverage   86.21%   86.19%   -0.02%     
==========================================
  Files           3        3              
  Lines         805      804       -1     
==========================================
- Hits          694      693       -1     
  Misses        111      111              

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