On macOS, don't use /var/folders for TMPDIR

This broke "nix-store --serve".

src/libmain/shared.cc

@@ -138,6 +138,14 @@ void initNix()
     struct timeval tv;
     gettimeofday(&tv, 0);
     srandom(tv.tv_usec);
+
+    /* On macOS, don't use the per-session TMPDIR (as set e.g. by
+       sshd). This breaks build users because they don't have access
+       to the TMPDIR, in particular in ‘nix-store --serve’. */
+#if __APPLE__
+    if (getuid() == 0 && hasPrefix(getEnv("TMPDIR"), "/var/folders/"))
+        unsetenv("TMPDIR");
+#endif
 }