Allow custom runners in extension's CI jobs #752

Job	Run time
Code Quality / Tidy Check	53s
Code Quality / Format Check	49s
Extension template (C API) / Generate matrix	32s
Extension template (C API) / linux_amd64	3m 43s
Extension template (C API) / windows_amd64	1m 28s
Extension template (C API) / wasm_eh	1m 18s
Extension template (C API) / matrix.duckdb_arch	0s
Extension template / Generate matrix	5s
Extension template (Rust) / Generate matrix	11s
Rust builds (using Delta extension) / Generate matrix	11s
Extension template / linux_amd64	1s
Extension template / linux_arm64	26s
Rust builds (using Delta extension) / linux_amd64	3m 12s
Extension template (Rust) / linux_amd64	1s
Rust builds (using Delta extension) / matrix.duckdb_arch	1s
Rust builds (using Delta extension) / matrix.duckdb_arch	1s
Extension template (Rust) / matrix.duckdb_arch	1s
Extension template (Rust) / matrix.duckdb_arch	1s
Extension template (Rust) / matrix.duckdb_arch	1s
Rust builds (using Delta extension) / matrix.duckdb_arch	1s
Extension template / matrix.duckdb_arch	1s
Extension template / matrix.duckdb_arch	1s
Extension template / matrix.duckdb_arch	1s
Deploy workflow dry run	-1s
	12m 58s

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