enable incremental LTO on macOS

[etan-status]

Set -cache_path_lto for incremental LTO on macOS.

https://clang.llvm.org/docs/ThinLTO.html

[arnetheduck]

this looks like something you may want to add to ~/.config/nim/nim.cfg instead - ie lto caches are not without their own problems in terms of cold compile time increases, hidden disk usage etc

[github-actions]

Unit Test Results

         9 files  ±0    1 098 suites  ±0   33m 10s ⏱️ + 1m 41s
  3 945 tests ±0    3 598 ✔️ ±0  347 💤 ±0  0 ❌ ±0 
16 048 runs  ±0  15 650 ✔️ ±0  398 💤 ±0  0 ❌ ±0 

Results for commit bda85c0. ± Comparison against base commit aa6c13c.