Set "meta.mainProgram" in Nix derivation
Nix outputs warning message when **user** calls "lib.getExe" on this
package.
---
evaluation warning: getExe: Package legit 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".