apptainer: singularity: use lib.concatMapAttrsStringSep

Use concatMapAttrsStringSep helper function from the standard library
instead of the locally-improvised one.

pkgs/applications/virtualization/singularity/generic.nix

@@ -102,10 +102,6 @@ in
 }@args:
 
 let
-  concatMapStringAttrsSep =
-    sep: f: attrs:
-    lib.concatMapStringsSep sep (name: f name attrs.${name}) (lib.attrNames attrs);
-
   addShellDoubleQuotes = s: lib.escapeShellArg ''"'' + s + lib.escapeShellArg ''"'';
 in
 (buildGoModule {
@@ -210,7 +206,7 @@ in
     patchShebangs --build "$configureScript" makeit e2e scripts mlocal/scripts
 
     # Patching the hard-coded defaultPath by prefixing the packages in defaultPathInputs
-    ${concatMapStringAttrsSep "\n" (fileName: originalDefaultPaths: ''
+    ${lib.concatMapAttrsStringSep "\n" (fileName: originalDefaultPaths: ''
       substituteInPlace ${lib.escapeShellArg fileName} \
         ${
           lib.concatMapStringsSep " \\\n  " (