Use a single test thread when running miri

[alamb]

Which issue does this PR close?

Testing to see if this closes #879

Theory is that something about the new version of MIRI uses more memory

Rationale for this change

MIRI is failing on master for reasons that are not reproducible (yet) locally for myself or @jimexist

What changes are included in this PR?

add --test-threads=1 command

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #891 (4645628) into master (898924f) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #891      +/-   ##
==========================================
- Coverage   82.45%   82.44%   -0.01%     
==========================================
  Files         168      168              
  Lines       48231    48231              
==========================================
- Hits        39767    39765       -2     
- Misses       8464     8466       +2     

Impacted Files	Coverage Δ
arrow/src/datatypes/datatype.rs	65.36% <0.00%> (-0.44%)	⬇️
parquet_derive/src/parquet_field.rs	66.59% <0.00%> (-0.23%)	⬇️
parquet/src/arrow/array_reader.rs	77.72% <0.00%> (-0.09%)	⬇️
parquet/src/encodings/encoding.rs	94.67% <0.00%> (+0.19%)	⬆️

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[alamb]

Single thread did not seem to help but skipping the cache did https://github.com/apache/arrow-rs/runs/4059773777?check_suite_focus=true

I'll open a new PR with just the right change