Fix Linux home resource path allocation

[Danmoreng]

Summary

• build the Linux user resource path directly with std::string
• remove the temporary asprintf allocation and mismatched delete
• avoid relying on the GNU-specific asprintf function for this path

Root cause

asprintf allocates with malloc, but queryResourceLocation released the result with delete. AddressSanitizer therefore aborted with an alloc-dealloc-mismatch whenever HOME was set during resource discovery.

Fixes #193.

Validation

• compiled the changed MicroTeX source as part of a downstream C++20 application
• downstream unit tests pass
• downstream ASan/UBSan unit tests pass with halt_on_error=1