fix: locate sdkroot required since big sur

[baszalmstra]

Apparently, since macOS Big Sur the location of system libraries moved. We now need to explicitly specify where to find these. I used the same technique used by Rust to locate this path.

Closes #361

[codecov]

Codecov Report

Merging #362 (58ff15c) into main (72dd828) will decrease coverage by 0.09%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #362      +/-   ##
==========================================
- Coverage   82.65%   82.56%   -0.10%     
==========================================
  Files         263      264       +1     
  Lines       15217    15234      +17     
==========================================
  Hits        12578    12578              
- Misses       2639     2656      +17     

Impacted Files	Coverage Δ
crates/mun_codegen/src/apple.rs	0.00% <0.00%> (ø)
crates/mun_codegen/src/linker.rs	25.33% <0.00%> (-1.43%)	⬇️
crates/mun_target/src/spec/apple_base.rs	69.23% <0.00%> (+9.23%)	⬆️

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