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/stdlib/ttimes.nim

@@ -114,7 +114,7 @@ suite "ttimes":
   # Generate tests for multiple timezone files where available
   # Set the TZ env var for each test
   when defined(linux) or defined(macosx):
-    const tz_dir = "/usr/share/zoneinfo"
+    let tz_dir = getEnv("TZDIR", "/usr/share/zoneinfo")
     const f = "yyyy-MM-dd HH:mm zzz"
     
     let orig_tz = getEnv("TZ")