Fix errors identified by R vendoring update

[paleolimbot]

A compiler warning:

geoarrow.c:1105:11: warning: unused variable 'chars_written' [-Wunused-variable]
 1105 |   int64_t chars_written = GeoArrowMetadataSerializeInternal(metadata_view, metadata);
      |           ^~~~~~~~~~~~~
geoarrow.c:1135:13: warning: unused variable 'chars_written' [-Wunused-variable]
 1135 |     int64_t chars_written = GeoArrowMetadataSerializeInternal(metadata_view, out);
      |             ^~~~~~~~~~~~~

...and ensure that running GEOARROW_BUNDLE=ON works with cmake --install (currently, the updates enabling a proper CMake install for a non-bundled version make this error).