Creating `TimeZone` from string allocates unnecessarily

```julia
julia> using TimeZones

julia> TimeZone("UTC");  # populate cache

julia> @allocations TimeZone("UTC")  
2
```
`UTC` is a `FixedTimeZone`, which is already in the cache, and a `FixedTimeZone` is `isbits`... but we still allocate twice (when we should not need to allocate at all)