fix(nix): set meta.mainProgram field (#634)

`meta.mainProgram` must be set otherwise `lib.getExe` assumes that the
package name is `television` and home manager shell integrations are
broken

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flake.nix

@@ -45,6 +45,9 @@
       in {
         packages.default = naersk'.buildPackage {
           src = ./.;
+          meta = {
+            mainProgram = "tv";
+          };
         };
         apps = {
           default = flake-utils.lib.mkApp {