MINOR: [R][CI] Guard against missing reticulate

[assignUser]

This caused some build failures where reticulate was not available.
@nealrichardson could you check this out?

[nealrichardson]

@github-actions crossbow submit opensuse

[github-actions]

Revision: 1d580f5

Submitted crossbow builds: ursacomputing/crossbow @ actions-1901

Task	Status
test-r-rstudio-r-base-4.1-opensuse15
test-r-rstudio-r-base-4.1-opensuse42

[nealrichardson]

Integration failure is unrelated

[ursabot]

Benchmark runs are scheduled for baseline = f7bccc5 and contender = cbf7d76. cbf7d76 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] test-mac-arm
[Failed ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.25% ⬆️0.04%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/579| cbf7d76a ec2-t3-xlarge-us-east-2>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-test-mac-arm/builds/567| cbf7d76a test-mac-arm>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/565| cbf7d76a ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/577| cbf7d76a ursa-thinkcentre-m75q>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/578| f7bccc51 ec2-t3-xlarge-us-east-2>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-test-mac-arm/builds/566| f7bccc51 test-mac-arm>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/564| f7bccc51 ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/576| f7bccc51 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