Collision in python packages due to test caches

[azazel75]

Issue description

Installing a derivation that has some python3.6 packages as buildInputs, it fails with the following error:

building '/nix/store/30akhvz6d921h8kj30d2m1qqvm86bls0-python3-3.6.6-env.drv'...
collision between `/nix/store/j9zhsaci7jw4z0ki1nwchvh7pra7dars-python3.6-cairocffi-0.8.1/lib/python3.6/site-packages/.pytest_cache/v/cache/nodeids' and `/nix/store/hnwfl6f04nvhgg733iyjaqs5sf6q9n5c-python3.6-tinycss-0.3/lib/python3.6/site-packages/.pytest_cache/v/cache/nodeids'
builder for '/nix/store/30akhvz6d921h8kj30d2m1qqvm86bls0-python3-3.6.6-env.drv' failed with exit code 25
error: build of '/nix/store/30akhvz6d921h8kj30d2m1qqvm86bls0-python3-3.6.6-env.drv' failed

It seems that some unneeded paths have find their way into the packages (the .pytest_cache/... entry). Those are just subproducts of the testing machinery.

I wasn't unable to find if these packages are obsolete as i'm using unstable

Steps to reproduce

I'm trying to reduce my code to something short that shows the error, but I failed until now, sorry... newbie here... I'll update this issue asap

Technical details

• system: "x86_64-linux"
• host os: Linux 4.17.5, NixOS, 18.09pre145679.dae9cf6106d (Jellyfish)
• multi-user?: yes
• sandbox: no
• version: nix-env (Nix) 2.0.4
• channels(root): "nixos-18.09pre147850.2428f5dda13"
• channels(azazel): ""
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

[FRidh]

Duplicate of #40273. This should be fixed on master and on the latest channel.

[FRidh]

Seems like it is still happening. Reopening #40273 instead.