mlir/ods-gen: use bash from env in shell script

On macOS, hardcoding the path of bash leads to a failure in executing
update_core_linalg_named_ops.sh, due to an old version of bash being
present in that location.

Signed-off-by: Ramkumar Ramachandra <r@artagnon.com>

Differential Revision: https://reviews.llvm.org/D139942