Simplify getenv call using cstdlib

[ferdymercury]

Description

Partially reverts c0a4dfe#diff-9e2485d191b0ee5c98436db2e99b854b71c96f67cb1d8b42870744ffbbe31acc

Is motivated by root-project/root#19864

Type of change

Simplification

Testing

see ROOT CI PR

Checklist

• I have read the contribution guide recently

[Vipul-Cariappa]

LGTM, If the CI is green.

[Vipul-Cariappa]

CI isn't happy. Looks like double free? (reference: https://github.com/compiler-research/CppInterOp/actions/runs/17621310446/job/50070564052?pr=709#step:12:725).

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[Vipul-Cariappa]

There might be some chances that the failures are not introduced by this PR; instead, it is due to the thread-safety PR. Will need to do some local testing to figure it out.
Reason for my speculation: This might be caused by the thread-safety PR.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[vgvassilev]

LGTM!

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[mcbarton]

Its not at all clear why the error is happening in the ci about frontend::IndexHeaderMap for llvm=20. That section of code shouldn't even be compiled for llvm=20 (see https://github.com/ferdymercury/CppInterOp/blob/87c288becb93ba23b3f6b818cdc80c93b23c69e2/lib/CppInterOp/Paths.cpp#L195) .