runtime: TimeZoneInfo.ToSerializedString/FromSerializedString do not round trip on Unix

The following test passes on Windows but fails on Unix:

public static IEnumerable<object[]> SystemTimeZonesTestData()
{
    foreach (TimeZoneInfo tz in TimeZoneInfo.GetSystemTimeZones())
    {
        yield return new object[] { tz };
    }
}

[Theory]
[MemberData(nameof(SystemTimeZonesTestData))]
public static void ToSerializedString_FromSerializedString_RoundTrips(TimeZoneInfo timeZone)
{
    string serialized = timeZone.ToSerializedString();
    TimeZoneInfo deserializedTimeZone = TimeZoneInfo.FromSerializedString(serialized);
    Assert.Equal(timeZone, deserializedTimeZone);
    Assert.Equal(serialized, deserializedTimeZone.ToSerializedString());
}

I believe it’s due to how GetAdjustmentRules is implemented on Unix: https://github.com/dotnet/coreclr/blob/0a11492d52faa85c551761f8390be5de9d74e5ec/src/mscorlib/src/System/TimeZoneInfo.Unix.cs#L131-L153

vs. on Windows: https://github.com/dotnet/coreclr/blob/0a11492d52faa85c551761f8390be5de9d74e5ec/src/mscorlib/src/System/TimeZoneInfo.Win32.cs#L87

See dotnet/corefx#14795

Also, when you pass an AdjustmentRule[] array to TimeZoneInfo.CreateCustomTimeZone to create a custom instance of TimeZoneInfo, calling GetAdjustmentRules on Windows will always give you a cloned copy of the same adjustment rules, whereas on Unix it looks like it will modify the adjustment rules that are returned.