Redirect some Nix command outputs to a pager

Make `nix search`, `nix flake show` and `nix flake metadata` pipe their
output into a pager to make their result easier to visualise.

src/nix/flake.cc

@@ -167,6 +167,8 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
         auto lockedFlake = lockFlake();
         auto & flake = lockedFlake.flake;
 
+        RunPager pager;
+
         if (json) {
             nlohmann::json j;
             if (flake.description)
@@ -1128,6 +1130,7 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
             Activity act(*logger, lvlInfo, actUnknown,
                 fmt("evaluating '%s'", concatStringsSep(".", attrPathS)));
 
+            std::optional<RunPager> pager = json ? std::nullopt : std::optional<RunPager>(RunPager());
             try {
                 auto recurse = [&]()
                 {

src/nix/search.cc

@@ -63,6 +63,7 @@ struct CmdSearch : InstallableValueCommand, MixJSON
 
     void run(ref<Store> store, ref<InstallableValue> installable) override
     {
+        RunPager pager;
         settings.readOnlyMode = true;
         evalSettings.enableImportFromDerivation.setDefault(false);