MINOR: [R] Unskip acero tests

[nealrichardson]

#34844 inadvertently resulted in skip_if_not_available("acero") to always skip.

[nealrichardson]

Latest run on main: https://github.com/apache/arrow/actions/runs/4634176275/jobs/8200123447#step:7:5348

...
'test-dplyr-arrange.R:20:1' [skip]
'test-dplyr-collapse.R:23:1' [skip]
'test-dplyr-count.R:20:1' [skip]
'test-dplyr-distinct.R:20:1' [skip]
'test-dplyr-filter.R:21:1' [skip]
'test-dplyr-funcs-conditional.R:21:1' [skip]
'test-dplyr-funcs-datetime.R:24:1' [skip]
'test-dplyr-funcs-math.R:20:1' [skip]
'test-dplyr-funcs-string.R:19:1' [skip]
'test-dplyr-funcs-type.R:22:1' [skip]
...

Here: https://github.com/apache/arrow/actions/runs/4632377834/jobs/8196366469?pr=34943#step:7:5349

...
'test-dplyr-arrange.R:26:3' [success]
'test-dplyr-arrange.R:26:3' [success]
'test-dplyr-arrange.R:32:3' [success]
'test-dplyr-arrange.R:32:3' [success]
'test-dplyr-arrange.R:38:3' [success]
'test-dplyr-arrange.R:38:3' [success]
'test-dplyr-arrange.R:44:3' [success]
'test-dplyr-arrange.R:44:3' [success]
'test-dplyr-arrange.R:50:3' [success]
'test-dplyr-arrange.R:50:3' [success]
...

[ursabot]

Benchmark runs are scheduled for baseline = 7526df9 and contender = 14e9e3c. 14e9e3c 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
[Finished ⬇️2.73% ⬆️0.8%] test-mac-arm
[Finished ⬇️0.26% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.49% ⬆️0.18%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] 14e9e3cb ec2-t3-xlarge-us-east-2
[Finished] 14e9e3cb test-mac-arm
[Finished] 14e9e3cb ursa-i9-9960x
[Finished] 14e9e3cb ursa-thinkcentre-m75q
[Finished] 7526df9a ec2-t3-xlarge-us-east-2
[Finished] 7526df9a test-mac-arm
[Finished] 7526df9a ursa-i9-9960x
[Finished] 7526df9a 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