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ARROW-11691: [Developer][CI] Provide a consolidated .env file for benchmark-relevant environment variables

This change will allow Arrow Contributors to add and update env vars used by benchmark builds that use conbench for running benchmarks using these steps:

1. Create `apache/arrow` PR
2. Update or add env var value in `benchmarks.env`
3. Add `@ursabot please benchmark` comment to PR
4. Once benchmark builds are done, benchmark results can be viewed via compare/runs links in the PR comment where
- baseline = PR base HEAD commit with default (master branch version) `/dev/conbench_envs/benchmarks.env`
- contender = PR branch HEAD commit with overridden `/dev/conbench_envs/benchmarks.env`

Closes #10667 from ElenaHenderson/ARROW-11691-benchmark-env

Lead-authored-by: ElenaHenderson <elenavhenderson@gmail.com>
Co-authored-by: Elena Henderson <elena@e.local>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>