Avoid GNU getcwd extension behavior

GNU `getcwd` can allocate a buffer if passed `NULL` for it. This is an
extension which is e.g., not recognized by clang-tidy-19's
`StdCLibraryFunctions` check[^1] which emits a diagnostic on a violated
precondition `buf != NULL`,

```
The 1st argument to 'getcwd' is NULL but should not be NULL [clang-analyzer-unix.StdCLibraryFunctions,-warnings-as-errors]
[build]  3987 |     std::unique_ptr<char, decltype(&std::free)> buffer{::getcwd(NULL, 0), std::free};
```

This patch modifies this use of `getcwd` with this extension behavior to
instead use `get_current_dir_name` which is also a GNU extension, but
does not trigger this diagnostic.

[^1]: https://clang.llvm.org/docs/analyzer/checkers.html#unix-stdclibraryfunctions-c

Author: bbannier

include/ghc/filesystem.hpp

@@ -4229,7 +4229,7 @@ GHC_INLINE path current_path(std::error_code& ec)
     }
     return path(std::wstring(buffer.get()), path::native_format);
 #elif defined(__GLIBC__)
-    std::unique_ptr<char, decltype(&std::free)> buffer { ::getcwd(NULL, 0), std::free };
+    std::unique_ptr<char, decltype(&std::free)> buffer { ::get_current_dir_name(), std::free };
     if (buffer == nullptr) {
         ec = detail::make_system_error();
         return path();