MINOR: [R] Fix compiler warning/CMD check NOTE when compiling with ARROW_R_WITH_ENGINE

[paleolimbot]

After ARROW-16033 (#12721) we get this compiler warning when compiling with ARROW_R_WITH_ENGINE:

   compute-exec.cpp:304:17: warning: 'Init' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
     arrow::Status Init(const std::shared_ptr<arrow::Schema>& schema) {
                   ^
   /Users/deweydunnington/.r-arrow-dev-build/dist/include/arrow/compute/exec/options.h:153:18: note: overridden virtual function is here
     virtual Status Init(const std::shared_ptr<Schema>& schema) = 0;
                    ^
   1 warning generated.

This PR just adds the requisite override.

[ursabot]

Benchmark runs are scheduled for baseline = 489aada and contender = d197ad3. d197ad3 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 ⬇️0.17% ⬆️0.08%] test-mac-arm
[Failed ⬇️2.14% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.04% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/478| d197ad31 ec2-t3-xlarge-us-east-2>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-test-mac-arm/builds/463| d197ad31 test-mac-arm>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/464| d197ad31 ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/473| d197ad31 ursa-thinkcentre-m75q>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/477| 489aada5 ec2-t3-xlarge-us-east-2>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-test-mac-arm/builds/462| 489aada5 test-mac-arm>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/463| 489aada5 ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/472| 489aada5 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