Update flake.lock for the latest rustc and set meta.mainProgram to make lib.getExe warning go away.

       > error: rustc 1.82.0 is not supported by the following packages:
       >   [email protected] requires rustc 1.83
       >   [email protected] requires rustc 1.83
       >   [email protected] requires rustc 1.83

evaluation warning: getExe: Package "redlib-0.36.0" does not have the meta.mainProgram attribute. We'll assume that the main program has the same name for now, but this behavior is deprecated, because it leads to surprising errors when the assumption does not hold. If the package has a main program, please set meta.mainProgram in its definition to make this warning go away. Otherwise, if the package does not have a main program, or if you don't control its definition, use getExe' to specify the name to the program, such as lib.getExe' foo "bar".