Run the builds in a daemon-controled directory

Instead of running the builds under
`$TMPDIR/{unique-build-directory-owned-by-the-build-user}`, run them
under `$TMPDIR/{unique-build-directory-owned-by-the-daemon}/{subdir-owned-by-the-build-user}`
where the build directory is only readable and traversable by the daemon user.

This achieves two things:

1. It prevents builders from making their build directory world-readable
   (or even writeable), which would allow the outside world to interact
   with them.
2. It prevents external processes running as the build user (either
   because that somehow leaked, maybe as a consequence of 1., or because
   `build-users` isn't in use) from gaining access to the build
   directory.

src/libutil/file-system.cc

@@ -412,6 +412,11 @@ void deletePath(const fs::path & path)
     deletePath(path, dummy);
 }
 
+void createDir(const Path &path, mode_t mode)
+{
+    if (mkdir(path.c_str(), mode) == -1)
+        throw SysError("creating directory '%1%'", path);
+}
 
 Paths createDirs(const Path & path)
 {