executors: use shutil.which instead of deprecated distutils

[quantum5]

See PEP 632, which suggests replacing distutils.spawn.find_executable with shutil.which.

[codecov-commenter]

Codecov Report

Merging #949 (04bc78c) into master (9704d41) will decrease coverage by 0.10%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #949      +/-   ##
==========================================
- Coverage   86.54%   86.44%   -0.11%     
==========================================
  Files         138      138              
  Lines        4736     4735       -1     
==========================================
- Hits         4099     4093       -6     
- Misses        637      642       +5     

Impacted Files	Coverage Δ
dmoj/executors/shell_executor.py	0.00% <0.00%> (ø)
dmoj/executors/base_executor.py	85.37% <100.00%> (-0.05%)	⬇️
dmoj/cptbox/tracer.py	83.27% <0.00%> (-1.82%)	⬇️
dmoj/result.py	89.33% <0.00%> (-1.34%)	⬇️
dmoj/graders/interactive.py	73.86% <0.00%> (+1.13%)	⬆️

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[quantum5]

retest this please

[Xyene]

LGTM