tzdb support in libstdcxx 14

[h-vetinari]

GCC 14 finally finished <chrono> support (search for P0355R7 here).

We'll need to set _GLIBCXX_ZONEINFO_DIR=$PREFIX/share/zoneinfo (see here), to point to our own tzdata, assuming relocation works correctly for the embedded path there. Otherwise we could patch the zoneinfo_dir_override function to directly load the path from $CONDA_PREFIX.