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

[nealrichardson]

R CMD check now raises a NOTE after my previous fix (f49fbda):

* 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 🤷

[ursabot]

Benchmark runs are scheduled for baseline = 5b653ee and contender = b264dca. b264dca is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.74% ⬆️0.0%] test-mac-arm
[Finished ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.16% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] b264dca5 ec2-t3-xlarge-us-east-2
[Finished] b264dca5 test-mac-arm
[Finished] b264dca5 ursa-i9-9960x
[Finished] b264dca5 ursa-thinkcentre-m75q
[Finished] 5b653ee2 ec2-t3-xlarge-us-east-2
[Failed] 5b653ee2 test-mac-arm
[Finished] 5b653ee2 ursa-i9-9960x
[Finished] 5b653ee2 ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java