nix shell: reflect command line order in PATH order

Prior to this change, Nix would prepend every installable to the PATH
list in order to ensure that installables appeared before the current
PATH from the ambient environment.

With this change, all the installables are still prepended to the PATH,
but in the same order as they appear on the command line. This means
that the first of two packages that expose an executable `hello` would
appear in the PATH first, and thus be executed first.

See the test in the prior commit for a more concrete example.

src/libcmd/installables.cc

@@ -729,6 +729,20 @@ StorePathSet Installable::toStorePathSet(
     return outPaths;
 }
 
+StorePaths Installable::toStorePaths(
+    ref<Store> evalStore,
+    ref<Store> store,
+    Realise mode, OperateOn operateOn,
+    const Installables & installables)
+{
+    StorePaths outPaths;
+    for (auto & path : toBuiltPaths(evalStore, store, mode, operateOn, installables)) {
+        auto thisOutPaths = path.outPaths();
+        outPaths.insert(outPaths.end(), thisOutPaths.begin(), thisOutPaths.end());
+    }
+    return outPaths;
+}
+
 StorePath Installable::toStorePath(
     ref<Store> evalStore,
     ref<Store> store,

src/libcmd/installables.hh

@@ -172,6 +172,13 @@ struct Installable
         OperateOn operateOn,
         const Installables & installables);
 
+    static std::vector<StorePath> toStorePaths(
+        ref<Store> evalStore,
+        ref<Store> store,
+        Realise mode,
+        OperateOn operateOn,
+        const Installables & installables);
+
     static StorePath toStorePath(
         ref<Store> evalStore,
         ref<Store> store,

src/nix/run.cc

@@ -114,15 +114,15 @@ struct CmdShell : InstallablesCommand, MixEnvironment
 
         setEnviron();
 
-        auto unixPath = tokenizeString<Strings>(getEnv("PATH").value_or(""), ":");
+        std::vector<std::string> pathAdditions;
 
         while (!todo.empty()) {
             auto path = todo.front();
             todo.pop();
             if (!done.insert(path).second) continue;
 
             if (true)
-                unixPath.push_front(store->printStorePath(path) + "/bin");
+                pathAdditions.push_back(store->printStorePath(path) + "/bin");
 
             auto propPath = CanonPath(store->printStorePath(path)) + "nix-support" + "propagated-user-env-packages";
             if (auto st = accessor->maybeLstat(propPath); st && st->type == SourceAccessor::tRegular) {
@@ -131,7 +131,10 @@ struct CmdShell : InstallablesCommand, MixEnvironment
             }
         }
 
-        setenv("PATH", concatStringsSep(":", unixPath).c_str(), 1);
+        auto unixPath = tokenizeString<Strings>(getEnv("PATH").value_or(""), ":");
+        unixPath.insert(unixPath.begin(), pathAdditions.begin(), pathAdditions.end());
+        auto unixPathString = concatStringsSep(":", unixPath);
+        setenv("PATH", unixPathString.c_str(), 1);
 
         Strings args;
         for (auto & arg : command) args.push_back(arg);