Allow passing chrome binary path via environment variable

[euporos]

I've been exploring cuic on NixOS, which doesn't adhere to the standard file system hierarchy, making the hard-coded paths in cuic.chrome problematic. This PR introduces the ability to specify the path to the Chrome binary using an environment variable.

This feature isn't just beneficial for NixOS users. Allowing users to point to a specific Chrome binary can be useful for testing against upcoming, unreleased versions of Chrome or pinning a specific version for consistent testing (e.g., using the Nix package manager on MacOS).