Crates with build.rs reading env CARGO_PKG_README broken

[p1k0chu]

Description

crate2nix breaks crates that have build.rs that reads env variable CARGO_PKG_README
specifically this crate doesn't build with crate2nix, but builds normally

Steps to Reproduce

1. create empty project and add rusqlite_migration as dependency
2. crate2nix generate
3. nix-build -f Cargo.nix rootCrate.build fails

Expected Behavior

I expected the build to succeed, as the crate builds without issues with regular cargo build.

Actual Behavior

in rusqlite_migration's build.rs, CARGO_PKG_README is an empty string, so it fails because that file doesn't exist

Environment

• crate2nix version: 0.15.0
• Nix version: 2.31.3
• OS: 6.18.13 x86_64 GNU/Linux, NixOS

Reproducible Test Case

#441