sbom4files: Set meta.mainProgram in python3Packages.sbom4files instead

Quote from NixOS 25.11 release notes:

> meta.mainProgram is now used to determine the NIX_MAIN_PROGRAM environment variable. This means that changing it can now lead to a package rebuild.

Make sbom4files and python3Packages.sbom4files the same derivation again.

pkgs/by-name/sb/sbom4files/package.nix

@@ -2,6 +2,4 @@
   python3Packages,
 }:
 
-(python3Packages.toPythonApplication python3Packages.sbom4files).overrideAttrs (previousAttrs: {
-  meta.mainProgram = "sbom4files";
-})
+python3Packages.toPythonApplication python3Packages.sbom4files

pkgs/development/python-modules/sbom4files/default.nix

@@ -37,6 +37,7 @@ buildPythonPackage rec {
     description = "SBOM generator for files within a directory";
     homepage = "https://github.com/anthonyharrison/sbom4files";
     license = lib.licenses.asl20;
+    mainProgram = "sbom4files";
     maintainers = [ ];
   };
 }