URL Literals are legacy syntax which has been long disallowed in NixOS/nixpkgs,
since Lix release 2.92 they have been deprecated in Lix.
See <https://lix.systems/blog/2025-01-18-lix-2.92-release/>.
Using the normal string syntax makes this flake compatible with both Nix
implementations without any special flags.
Signed-off-by: max <github@p2502.net>
Replacing `etc/shell.nix`, this is a [flake](https://nixos.wiki/wiki/Flakes) based on the OpenROAD Nix derivation used for OpenLane 2. With Nix installed with flakes enabled, getting an OpenROAD environment is as simple as typing `nix develop`.
Also fixes issue with versions of OpenROAD built in the dev environments failing to launch the GUI.
Signed-off-by: Mohamed Gaber <donn@efabless.com>
