flake.nix: make checks friendly for non-x86_64-linux systems

(cherry picked from commit 9ebed2c8ea)

Without the slightly backwards incompatible changes,
see https://github.com/NixOS/nixpkgs/pull/317772#discussion_r1632475729

flake.nix

@@ -44,13 +44,14 @@
           );
       });
 
-      checks.x86_64-linux = {
-        tarball = jobs.x86_64-linux.tarball;
+      checks = forAllSystems (system: {
+        tarball = jobs.${system}.tarball;
+      } // lib.optionalAttrs (self.legacyPackages.${system}.stdenv.isLinux) {
         # Test that ensures that the nixosSystem function can accept a lib argument
         # Note: prefer not to extend or modify `lib`, especially if you want to share reusable modules
         #       alternatives include: `import` a file, or put a custom library in an option or in `_module.args.<libname>`
         nixosSystemAcceptsLib = (self.lib.nixosSystem {
-          pkgs = self.legacyPackages.x86_64-linux;
+          pkgs = self.legacyPackages.${system};
           lib = self.lib.extend (final: prev: {
             ifThisFunctionIsMissingTheTestFails = final.id;
           });
@@ -66,7 +67,7 @@
             })
           ];
         }).config.system.build.toplevel;
-      };
+      });
 
       htmlDocs = {
         nixpkgsManual = jobs.x86_64-linux.manual;