Fix arm tests on Python PostCommit

[damccorm]

This fixes the Python arm postcommit and enables it normally. It does this by forcing the jobs to use custom arm containers. Example passing run - https://github.com/apache/beam/actions/runs/6405410458/job/17387943613

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[codecov]

Codecov Report

Merging #28740 (5177fc5) into master (a46bc12) will decrease coverage by 0.01%.
Report is 40 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #28740      +/-   ##
==========================================
- Coverage   72.22%   72.22%   -0.01%     
==========================================
  Files         684      684              
  Lines      101241   101230      -11     
==========================================
- Hits        73119    73110       -9     
+ Misses      26544    26542       -2     
  Partials     1578     1578              

Flag	Coverage Δ
python	82.76% <ø> (+<0.01%)	⬆️

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[damccorm]

R: @tvalentyn

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[tvalentyn]

Fantastic, thanks, Danny!