Add substrait label to PRs

[alamb]

Which issue does this PR close?

NA

Rationale for this change

I find myself trying to find PRs in need of review -- having label categorization helps me do so

What changes are included in this PR?

label any PR that modifies a file in datafusion/subtrait with the substrait label. Example

https://github.com/apache/arrow-datafusion/pulls?q=is%3Apr+is%3Aopen+label%3Asubstrait

Are these changes tested?

No (I don't know how to test them other than "live")

Are there any user-facing changes?

No, dev process only

[jackwener]

Nice job to me, thanks @alamb

[ursabot]

Benchmark runs are scheduled for baseline = e2daee9 and contender = 195eccf. 195eccf is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
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