Verify $HOME exists and is owned by current user in getHome()

Useful because a default `sudo` on darwin doesn't clear `$HOME`, so things like `sudo nix-channel --list`
will surprisingly return the USER'S channels, rather than `root`'s.

Other counterintuitive outcomes can be seen in this PR description:
  #6622

src/libutil/util.cc

@@ -574,6 +574,14 @@ Path getHome()
     static Path homeDir = []()
     {
         auto homeDir = getEnv("HOME");
+        if (homeDir) {
+            // Only use $HOME if it exists and is owned by the current user.
+            struct stat st = {};
+            if (stat(homeDir.c_str(), &st) || st.uid_t != geteuid()) {
+                // Couldn't stat $HOME, or the location wasn't owned by the current user
+                homeDir.reset();
+            }
+        }
         if (!homeDir) {
             std::vector<char> buf(16384);
             struct passwd pwbuf;