Devenv updates

[mpscholten]

This PR removes the need for the --impure flag when using devenv as suggested in cachix/devenv#1018

Requires an update of the projects .envrc:

# Old:
use flake . --impure --accept-flake-config

# New:
if ! use flake . --accept-flake-config --override-input ihp/devenv-root "file+file://"<(printf %s "$PWD")
then
    echo "devenv could not be built. The devenv environment was not loaded. Make the necessary changes to devenv.nix and hit enter to try again." >&2
fi