Use CARGO_CFG_TARGET_OS instead of target_os cfg var

[lu-zero]

target_os reports the target used to build the build.rs script, while it is needed to check which is the target used to build the crate itself.

Partially unbreaks cross-building, it covers only macos to linux.

[semanticdiff-com[bot]]

Review changes with SemanticDiff.

Analyzed 1 of 1 files.

Overall, the semantic diff is 43% smaller than the GitHub diff.

	Filename	Status
:heavy_check_mark:	hwlocality-sys/build.rs	42.42% smaller

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f6e9a92) 65.28% compared to head (31b1329) 66.53%. Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #93 +/- ## ========================================== + Coverage 65.28% 66.53% +1.25% ========================================== Files 43 42 -1 Lines 4632 4076 -556 ========================================== - Hits 3024 2712 -312 + Misses 1608 1364 -244 ``` | [Flag](https://app.codecov.io/gh/HadrienG2/hwlocality/pull/93/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hadrien+G.) | Coverage Δ | | |---|---|---| | [ubuntu-latest](https://app.codecov.io/gh/HadrienG2/hwlocality/pull/93/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hadrien+G.) | `66.53% <ø> (ø)` | | | [windows-latest](https://app.codecov.io/gh/HadrienG2/hwlocality/pull/93/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hadrien+G.) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hadrien+G.#carryforward-flags-in-the-pull-request-comment) to find out more.

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[HadrienG2]

Thanks for the fix!