nix run: Use packages/legacyPackages as fallback if there is no app definition

'nix run' will try to run $out/bin/<name>, where <name> is the
derivation name (excluding the version). This often works well:

  $ nix run nixpkgs#hello
  Hello, world!

  $ nix run nix -- --version
  nix (Nix) 2.4pre20200626_adf2fbb

  $ nix run patchelf -- --version
  patchelf 0.11.20200623.e61654b

  $ nix run nixpkgs#firefox -- --version
  Mozilla Firefox 77.0.1

  $ nix run nixpkgs#gimp -- --version
  GNU Image Manipulation Program version 2.10.14

though not always:

  $ nix run nixpkgs#git
  error: unable to execute '/nix/store/kp7wp760l4gryq9s36x481b2x4rfklcy-git-2.25.4/bin/git-minimal': No such file or directory

1 files changed, 5 insertions, 2 deletions

diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 17df29fdb..847985ea3 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -243,11 +243,14 @@ struct CmdFlakeCheck : FlakeCommand
 
         auto checkApp = [&](const std::string & attrPath, Value & v, const Pos & pos) {
             try {
+                #if 0
+                // FIXME
                 auto app = App(*state, v);
                 for (auto & i : app.context) {
                     auto [drvPathS, outputName] = decodeContext(i);
                     store->parseStorePath(drvPathS);
                 }
+                #endif
             } catch (Error & e) {
                 e.addPrefix(fmt("while checking the app definition '" ANSI_BOLD "%s" ANSI_NORMAL "' at %s:\n", attrPath, pos));
                 throw;
@@ -544,9 +547,9 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
             Strings{templateName == "" ? "defaultTemplate" : templateName},
             Strings(attrsPathPrefixes), lockFlags);
 
-        auto cursor = installable.getCursor(*evalState, true);
+        auto [cursor, attrPath] = installable.getCursor(*evalState, true);
 
-        auto templateDir = cursor.first->getAttr("path")->getString();
+        auto templateDir = cursor->getAttr("path")->getString();
 
         assert(store->isInStore(templateDir));