feat: read installable paths from stdin

Resolves NixOS#7437 for new `nix` commands only.

If paths are also passed on the cli they will be combined with the ones
from standard input.

src/libcmd/installables.cc

@@ -1089,10 +1089,18 @@ void InstallablesCommand::prepare()
 
 Installables InstallablesCommand::load() {
     Installables installables;
-    if (_installables.empty() && useDefaultInstallables())
+    if (_installables.empty() && useDefaultInstallables() && isatty(STDIN_FILENO))
         // FIXME: commands like "nix profile install" should not have a
         // default, probably.
         _installables.push_back(".");
+
+    if (!isatty(STDIN_FILENO)) {
+        std::string word;
+        while (std::cin >> word) {
+            _installables.emplace_back(std::move(word));
+        }
+    }
+
     return parseInstallables(getStore(), _installables);
 }