testers.lycheeLinkCheck: Add online attribute for nix run

(cherry picked from commit c2ebd3ea60)

doc/build-helpers/testers.chapter.md

@@ -44,7 +44,13 @@ If the `moduleNames` argument is omitted, `hasPkgConfigModules` will use `meta.p
 
 Check that internal hyperlinks are correct, using the [`lychee` package](https://search.nixos.org/packages?show=lychee&type=packages&query=lychee).
 
-Only an offline check is performed, so that network access is not required.
+When building the check, only an offline check is performed, so that network access is not required.
+
+If you'd like to run the check with network access, the returned attribute `online` can be invoked with [`nix run`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-run) ([experimental](https://nixos.org/manual/nix/stable/contributing/experimental-features#xp-feature-nix-command)). For example:
+
+```shell
+nix run nixpkgs#lychee.tests.ok.online
+```
 
 :::{.example #ex-lycheelinkcheck}
 

pkgs/build-support/testers/lychee.nix

@@ -1,4 +1,4 @@
-deps@{ formats, lib, lychee, stdenv }:
+deps@{ formats, lib, lychee, stdenv, writeShellApplication }:
 let
 
   # See https://nixos.org/manual/nixpkgs/unstable/#tester-lycheeLinkCheck
@@ -23,6 +23,18 @@ let
         } // lib.optionalAttrs (finalAttrs.passthru.remapUrl != null) {
           remap = [ "${remapUrl} file://${finalAttrs.site}" ];
         } // extraConfig;
+        online = writeShellApplication {
+          name = "run-lychee-online";
+          runtimeInputs = [ finalAttrs.passthru.lychee ];
+          # Comment out to run shellcheck:
+          checkPhase = "";
+          text = ''
+            site=${finalAttrs.site}
+            configFile=${finalAttrs.configFile}
+            echo Checking links on $site
+            exec lychee --config $configFile $site "$@"
+          '';
+        };
       };
       buildCommand = ''
         echo Checking internal links on $site