nix-shell with coq+compcert will not start

[veprbl]

Describe the bug
Shell with coq and compcert will fail to start with a following message:

bash: COQPATH: unbound variable

This is an issue because compcert depends on coq so they would be always included to the shell started by nix-review. I noticed this issue while reviewing #73958.

I suspect this is caused by #72347.

To Reproduce
Steps to reproduce the behavior:

NIXPKGS_ALLOW_UNFREE=1 nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/814f9104d721dcbf267720d8964095626b45321b.tar.gz -p compcert -p coq

This does not reproduce on release-19.09

Expected behavior
Shell will start, shell prompt will appear.

Metadata
Please run nix run nixpkgs.nix-info -c nix-info -m and paste the result.

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:

cc @Ericson2314 @thoughtpolice @jwiegley @vbgl

[eraserhd]

I'm getting "CLASSPATH: unbound variable" since updating master an hour ago from last Sunday, and @hlolli reports "JAVA_HOME: unbound variable" started happening while working on #72331 eleven hours ago.