Fix tests on NixOS (#9209) (#9285)

* Replace `/bin/sleep` with just `sleep`, i.e. use environment variable
  `$PATH` to locate binary.
* Replace `/usr/share/zoneinfo` with `$TZDIR` when it is defined,
  fallback to hardcoded path otherwise. This is the same behavior that
  Glibc2 normally have, see man 3 tzset.

tests/async/tupcoming_async.nim

@@ -100,7 +100,7 @@ when defined(upcoming):
                                    {poStdErrToStdOut, poUsePath, poInteractive,
                                    poDemon})
       else:
-        var process = startProcess("/bin/sleep", "", ["1"], nil,
+        var process = startProcess("sleep", "", ["1"], nil,
                                    {poStdErrToStdOut, poUsePath})
       var fut = waitProcess(process)
       waitFor(fut or waitTimer(2000))