develop: Discard the input{Overrides,Updates} when getting bash

`nix develop` is getting bash from an (assumed existing) `nixpkgs`
flake. However, when doing so, it reuses the `lockFlags` passed to the
current flake, including the `--input-overrides` and `--input-update`
which generally don’t make sense anymore at that point (and trigger a
warning because of that)

Clear these overrides before getting the nixpkgs flake to get rid of the
warning.