Add mainProgram to xorg.xrandr

[Fryuni]

Description of changes

Fix warning described in #249169 (comment)

trace: warning: getExe: Package "xrandr-1.5.2" 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".

Things done

Added "xrandr" as the main program name for xorg.xrandr.

As mentioned on #219567 (comment) I think this serves as a workaround to remove the warning that is suddenly appearing for (to my understanding) any user using an xorg-based desktop environment that updated after #246386. But IMHO, the name of the executable should be included in this generated file

[Artturin]

The name of the executable is not easily known during the generation script (didnt check, just assuming) so overrides are needed

[nagy]

Thanks, can you follow the guidelines for the commit message at https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#commit-conventions ?

[Fryuni]

Sorry, I'm used to PRs getting squashed so the messages are not that important. Fixing it.

[Fryuni]

Ok, I clearly did something wrong when rebasing

[Fryuni]

Fixed, sorry for all the pings that happened to code owners because of that.