Swap order of attempts in getHome() to check syscall first and `$HO…

…ME` second if necessary

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

@@ -573,16 +573,22 @@ Path getHome()
 {
     static Path homeDir = []()
     {
-        auto homeDir = getEnv("HOME");
-        if (!homeDir) {
-            std::vector<char> buf(16384);
-            struct passwd pwbuf;
-            struct passwd * pw;
-            if (getpwuid_r(geteuid(), &pwbuf, buf.data(), buf.size(), &pw) != 0
-                || !pw || !pw->pw_dir || !pw->pw_dir[0])
-                throw Error("cannot determine user's home directory");
+        std::vector<char> buf(16384);
+        struct passwd pwbuf;
+        struct passwd * pw;
+
+        std::optional<std::string> homeDir = {};
+
+        if (getpwuid_r(geteuid(), &pwbuf, buf.data(), buf.size(), &pw) == 0
+            && pw && pw->pw_dir && pw->pw_dir[0])
             homeDir = pw->pw_dir;
-        }
+
+        if (!homeDir)
+            homeDir = getEnv("HOME");
+
+        if (!homeDir || !*homeDir[0])
+            throw Error("cannot determine user's home directory");
+
         return *homeDir;
     }();
     return homeDir;