fix update headers to download correct headers

[samansmink]

This allows automatically fetching the headers that are configured using TARGET_DUCKDB_VERSION.

So imagine I want to build an extension specifically for the latest unstable version of duckdb in pip which happens to be commit 5d02d69e5c. The process will be:

First set these variables in the makefile (or pass them to it)

• TARGET_DUCKDB_VERSION=5d02d69e5c
• USE_UNSTABLE_C_API=1
• DUCKDB_TEST_VERSION=1.1.4.dev4889

Then:

# installs duckdb==1.1.4.dev4889 and sqllogictestrunner through pip
make configure
# fetches the correct version of duckdb.h and duckdb_extension.h
make update_duckdb_headers
# Builds the extension of abi_type C_STRUCT_UNSTABLE
make debug
# Run the tests on the extension
make test_debug