But also, just to double check, I modified the vendored code and no difference:
CGO_LDFLAGS="-L$(pwd)/duckdb/src/include" CGO_CFLAGS="-I$(pwd)/duckdb/src/include" go build -ldflags '-extldflags " -lstdc++ -lm -lduckdb -static"'
# github.com/marcboeker/go-duckdb
vendor/github.com/marcboeker/go-duckdb/connection.go:4:10: fatal error: duckdb.h: No such file or directory
4 | #include "duckdb.h"
| ^~~~~~~~~~
compilation terminated.