arrow

MINOR: [R] Move tzdb loading out of .onLoad() to avoid a check NOTE

`R CMD check` now raises a NOTE after my previous fix (f49fbda3dffeaead7d192ec64bfd2a7cfc4172a3):

```
* checking R code for possible problems ... NOTE
File ‘arrow/R/arrow-package.R’:
  .onLoad calls:
    packageStartupMessage("The tzdb package is not installed. Timezones will not be available.")
See section ‘Good practice’ in '?.onAttach'.
```

Interestingly, the docs they point to say to use `packageStartupMessage()` like we are doing here. In any case, if we move it to a function that `.onLoad()` calls rather than having it directly in .onLoad, `check` doesn't find it 🤷

Closes #13104 from nealrichardson/tzdb-msg-2

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>