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/tioselectors.nim

@@ -163,9 +163,9 @@ elif not defined(windows):
 
     proc process_notification_test(): bool =
       var selector = newSelector[int]()
-      var process2 = startProcess("/bin/sleep", "", ["2"], nil,
+      var process2 = startProcess("sleep", "", ["2"], nil,
                            {poStdErrToStdOut, poUsePath})
-      discard startProcess("/bin/sleep", "", ["1"], nil,
+      discard startProcess("sleep", "", ["1"], nil,
                            {poStdErrToStdOut, poUsePath})
 
       selector.registerProcess(process2.processID, 0)