scripted-diff: rename GetSystemTimeInSeconds to GetTimeSeconds
This PR simply renames GetSystemTimeInSeconds to GetTimeSeconds, for uniformity amongst our time handling functions (GetTimeMillis, GetTimeMicros). I have a branch that does a chunk of GetTime() -> GetSystemTimeInSeconds (GetTimeSeconds) / GetTime<T> migration, so we can eventually remove the (2 year) deprecated GetTime().
However, splitting this off and doing the renaming first while the number of GetSystemTimeInSeconds instances is small seems worthwhile.