bug#42992: nix-channel refers to a non existent bash-minimal

[Brendan Tildesley]

I use nix for some libre software not available in guix yet. currently 
I'm getting the following error:

b@ui ~$ nix-channel --update

unpacking channels...
while setting up the build environment: executing 
'/gnu/store/pwcp239kjf7lnj5i4lkdzcfcxwcfyk72-bash-minimal-5.0.16/bin/bash': 
No such file or directory
builder for 
'/nix/store/vbcwq7qgz2372vhjg1mjzcmln5q2rihz-nixos-20.09pre239318.c59ea8b8a0e.drv' 
failed with exit code 1
error: build of 
'/nix/store/jimxjljq433dm5arwyps9yp0qap8n4rc-nixpkgs-20.09pre239803.909539e6a09.drv', 
'/nix/store/vbcwq7qgz2372vhjg1mjzcmln5q2rihz-nixos-20.09pre239318.c59ea8b8a0e.drv' 
failed
error: program 
'/gnu/store/ngk9ir0am97190hf4p9v0rwcvm8bcgn2-nix-2.3.6/bin/nix-env' 
failed with exit code 100
b@ui ~$
b@ui ~$ guix build nix
/gnu/store/ngk9ir0am97190hf4p9v0rwcvm8bcgn2-nix-2.3.6
b@ui ~$ rg bash-minimal 
/gnu/store/ngk9ir0am97190hf4p9v0rwcvm8bcgn2-nix-2.3.6
/gnu/store/ngk9ir0am97190hf4p9v0rwcvm8bcgn2-nix-2.3.6/share/nix/corepkgs/config.nix 

6:  shell = 
"/gnu/store/pwcp239kjf7lnj5i4lkdzcfcxwcfyk72-bash-minimal-5.0.16/bin/bash";
-------------------------------

I am unsure where this bash-minimal comes from. it is not the latest 
bash-minimal returned by guix build bash-minimal. Adding bash-minimal to 
nix's inputs didn't fix it.