Fix default argument attributes

[dkorpel]

Blocking dlang/dmd#22903

The value of getDefaultTZDatabaseDir() is used as a default argument in @safe functions, which was previously allowed, but with the bug fix applied gives the correct error:

  std/datetime/timezone.d(2083): Error: `@safe` function `std.datetime.systime.SysTime.__unittest_L6751_C11` cannot call `@system` function `std.datetime.timezone.PosixTimeZone.getDefaultTZDatabaseDir`
      static immutable(PosixTimeZone) getTimeZone(string name, string tzDatabaseDir = getDefaultTZDatabaseDir()) @trusted
                                                                                                             ^
  std/datetime/timezone.d(2044):        `std.datetime.timezone.PosixTimeZone.getDefaultTZDatabaseDir` is declared here
      private static string getDefaultTZDatabaseDir()
                            ^

[dlang-bot]

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