[minor] Clean up uneeded environment variables in CI

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

In the context of apache/arrow-rs#2149 I spent a lot of time reviewing github workflow definitions and so naturally I also started looking at the datafusion ones as well. IN my review, I found some leftover cruft from earlier times that I think is no longer necessary

What changes are included in this PR?

Remove unecessary environment variable definitions in Rust CI definitions

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #2989 (f53f8a9) into master (c7fa789) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2989   +/-   ##
=======================================
  Coverage   85.81%   85.81%           
=======================================
  Files         282      282           
  Lines       51531    51531           
=======================================
+ Hits        44219    44222    +3     
+ Misses       7312     7309    -3     

Impacted Files	Coverage Δ
datafusion/expr/src/logical_plan/plan.rs	77.77% <0.00%> (+0.34%)	⬆️
datafusion/core/src/physical_plan/metrics/value.rs	87.43% <0.00%> (+0.50%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us.

[alamb]

aaand all the CI checks passed without the variables 🎉

[ursabot]

Benchmark runs are scheduled for baseline = 193fc3b and contender = c179102. c179102 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