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:
  NixOS#6622

src/libutil/util.cc

@@ -574,6 +574,20 @@ Path getHome()
     static Path homeDir = []()
     {
         auto homeDir = getEnv("HOME");
+        if (homeDir) {
+            // Only use $HOME if doesn't exist or is owned by the current user.
+            struct stat st;
+            int result = stat(homeDir->c_str(), &st);
+            if (result != 0) {
+                if (errno != ENOENT) {
+                    warn("Couldn't stat $HOME ('%s') for reason other than not existing ('%d'), falling back to the one defined in the 'passwd' file", *homeDir, errno);
+                    homeDir.reset();
+                }
+            } else if (st.st_uid != geteuid()) {
+                warn("$HOME ('%s') is not owned by you, falling back to the one defined in the 'passwd' file", *homeDir);
+                homeDir.reset();
+            }
+        }
         if (!homeDir) {
             std::vector<char> buf(16384);
             struct passwd pwbuf;

tests/config.sh

@@ -4,6 +4,10 @@ source common.sh
 # Other tests (e.g. flake registry tests) could be writing to $HOME in parallel.
 export HOME=$TEST_ROOT/userhome
 
+# # If home didn't exist or wasn't owned by this user, nix would have reverted to
+# # using the homedir entry from /etc/passwd instead.
+# mkdir $HOME
+
 # Test that using XDG_CONFIG_HOME works
 # Assert the config folder didn't exist initially.
 [ ! -e "$HOME/.config" ]

tests/tarball.sh

@@ -3,6 +3,9 @@ source common.sh
 clearStore
 
 rm -rf $TEST_HOME
+# # If home didn't exist or wasn't owned by this user, nix would have reverted to
+# # using the homedir entry from /etc/passwd instead.
+# mkdir $TEST_HOME
 
 tarroot=$TEST_ROOT/tarball
 rm -rf $tarroot