TypeError: str expected, not int

[austin3dickey]

https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/3752#018ab479-f500-4212-a289-623884f53853/33-5025

Traceback (most recent call last):
  File "/var/lib/buildkite-agent/miniconda3/envs/arrow-commit/bin/conbench", line 5, in <module>
    from conbenchlegacy.cli import conbench
  File "/var/lib/buildkite-agent/miniconda3/envs/arrow-commit/lib/python3.8/site-packages/conbenchlegacy/cli.py", line 87, in <module>
    instance = benchmark()
  File "/var/lib/buildkite-agent/builds/ip-172-31-43-254-1/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/benchmarks/benchmarks/cpp_micro_benchmarks.py", line 107, in __init__
    os.environ["CONBENCH_PROJECT_PR_NUMBER"] = self.github_info["pr_number"]
  File "/var/lib/buildkite-agent/miniconda3/envs/arrow-commit/lib/python3.8/os.py", line 680, in __setitem__
    value = self.encodevalue(value)
  File "/var/lib/buildkite-agent/miniconda3/envs/arrow-commit/lib/python3.8/os.py", line 750, in encode
    raise TypeError("str expected, not %s" % type(value).__name__)
TypeError: str expected, not int

Offending code:

benchmarks/benchmarks/cpp_micro_benchmarks.py

Line 107 in 079a920

os.environ["CONBENCH_PROJECT_PR_NUMBER"] = self.github_info["pr_number"]

This only happens during PR benchmarks (i.e. the "ursabot please benchmark" workflow).