Fix hardcoded extension_name #145

Job	Run time
Extension template / Generate matrix	25s
Rust builds (using Delta extension) / Generate matrix	24s
Rust builds (using Delta extension) / Linux (linux_amd64, ubuntu:18.04, x64-linux)	20m 23s
Rust builds (using Delta extension) / Windows	0s
Rust builds (using Delta extension) / DuckDB-Wasm	0s
Rust builds (using Delta extension) / MacOS (osx_amd64, x86_64, x64-osx)	7m 20s
Rust builds (using Delta extension) / Linux (linux_amd64_gcc4, quay.io/pypa/manylinux2014_x86_64, x64-linux)	20m 21s
Rust builds (using Delta extension) / MacOS (osx_arm64, arm64, arm64-osx)	9m 1s
Rust builds (using Delta extension) / Linux (linux_arm64, ubuntu:18.04, arm64-linux)	21m 47s
Extension template / DuckDB-Wasm (wasm_mvp, wasm32-emscripten)	3m 25s
Extension template / MacOS (osx_amd64, x86_64, x64-osx)	3m 10s
Extension template / Linux (linux_amd64, ubuntu:18.04, x64-linux)	6m 13s
Extension template / DuckDB-Wasm (wasm_eh, wasm32-emscripten)	4m 0s
Extension template / MacOS (osx_arm64, arm64, arm64-osx)	1m 39s
Extension template / Linux (linux_amd64_gcc4, quay.io/pypa/manylinux2014_x86_64, x64-linux)	5m 41s
Extension template / DuckDB-Wasm (wasm_threads, wasm32-emscripten)	3m 54s
Extension template / Linux (linux_arm64, ubuntu:18.04, arm64-linux)	6m 42s
Extension template / Windows (windows_amd64, x64-windows-static-md)	20m 14s
Extension template / Windows (windows_amd64_rtools, x64-mingw-static)	42m 0s
	2h 56m 39s

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