bash: build with bash_cv_getcwd_malloc=yes

This should disable bash's broken builtin getcwd(), hopefully fixing errors like:

shell-init: error retrieving current directory: getcwd: cannot access parent directories: Bad file descriptor

This is based on https://github.com/NixOS/nixpkgs/commit/dff0ba38a243603534c99f2cf1af81c793cad98e.

Fixes #320 (closed)