Call duckdb custom calls in a subprocess (with a unique lib path)

[jonkeane]

So that one doesn't need to constantly need to rebuild duckdb switching between released and with-tpch-enabled versions:

See https://github.com/ursacomputing/arrowbench/pull/64#discussion_r772743721

This will be slightly less annoying once DuckDB releases a new release now that DuckDB can install extensions at runtime