Use `Threads.nthreads()` instead of `num_threads`

[codecov[bot]]

Codecov Report

Base: 86.17% // Head: 86.54% // Increases project coverage by +0.36% :tada:

Coverage data is based on head (09baba2) compared to base (430701e). Patch coverage: 85.57% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #163 +/- ## ========================================== + Coverage 86.17% 86.54% +0.36% ========================================== Files 13 13 Lines 998 1003 +5 ========================================== + Hits 860 868 +8 + Misses 138 135 -3 ``` | [Impacted Files](https://codecov.io/gh/JuliaLinearAlgebra/Octavian.jl/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra) | Coverage Δ | | |---|---|---| | [src/Octavian.jl](https://codecov.io/gh/JuliaLinearAlgebra/Octavian.jl/pull/163/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL09jdGF2aWFuLmps) | `50.00% <ø> (ø)` | | | [src/global\_constants.jl](https://codecov.io/gh/JuliaLinearAlgebra/Octavian.jl/pull/163/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL2dsb2JhbF9jb25zdGFudHMuamw=) | `6.00% <0.00%> (-6.00%)` | :arrow_down: | | [src/memory\_buffer.jl](https://codecov.io/gh/JuliaLinearAlgebra/Octavian.jl/pull/163/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL21lbW9yeV9idWZmZXIuamw=) | `75.00% <80.00%> (+1.08%)` | :arrow_up: | | [src/block\_sizes.jl](https://codecov.io/gh/JuliaLinearAlgebra/Octavian.jl/pull/163/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL2Jsb2NrX3NpemVzLmps) | `86.04% <91.42%> (-0.48%)` | :arrow_down: | | [src/matmul.jl](https://codecov.io/gh/JuliaLinearAlgebra/Octavian.jl/pull/163/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL21hdG11bC5qbA==) | `92.79% <91.50%> (+1.27%)` | :arrow_up: | | [src/init.jl](https://codecov.io/gh/JuliaLinearAlgebra/Octavian.jl/pull/163/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL2luaXQuamw=) | `85.18% <100.00%> (ø)` | | | [src/funcptrs.jl](https://codecov.io/gh/JuliaLinearAlgebra/Octavian.jl/pull/163/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL2Z1bmNwdHJzLmps) | `100.00% <0.00%> (+2.70%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra)

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