flake: Compile commit hash into program version — heartwood

Changes build.rs to take the GIT_HASH environment variable into account if git rev-parse is unable to determine the current commit hash.

 use std::process::Command;
 use std::env;
 fn main() {
     // Set a build-time `GIT_HEAD` env var which includes the commit id;

                 None
             }
         })
         .unwrap_or_else(|| String::from("unknown"));
         .unwrap_or(env::var("GIT_HEAD").unwrap_or("unknown".into()));
     println!("cargo:rustc-env=GIT_HEAD={hash}");
     println!("cargo:rustc-rerun-if-changed=.git/HEAD");

             # Additional darwin specific inputs can be set here
             pkgs.libiconv
           ];
         preBuild = lib.optionalString (self.shortRev or null != null) ''
           export GIT_HEAD=${self.shortRev}
         '';
       };
       # Build *just* the cargo dependencies, so we can reuse