judgeenv: properly specify a default in docker

[Riolku]

What we probably want to do here is have a default when we are in docker. Right now, if the model_file exists, the default is completely ignored.

[codecov-commenter]

Codecov Report

Merging #1027 (11ac821) into master (9551b06) will increase coverage by 2.98%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #1027 +/- ## ========================================== + Coverage 81.24% 84.23% +2.98%  ========================================== Files 140 140 Lines 4789 4789 ========================================== + Hits 3891 4034 +143  + Misses 898 755 -143 

Impacted Files	Coverage Δ
dmoj/judgeenv.py	56.84% <ø> (ø)
dmoj/judge.py	53.98% <0.00%> (+1.22%)	⬆️
dmoj/executors/java_executor.py	86.36% <0.00%> (+3.03%)	⬆️
dmoj/cptbox/compiler_isolate.py	55.31% <0.00%> (+6.38%)	⬆️
dmoj/cptbox/tracer.py	76.32% <0.00%> (+18.02%)	⬆️
dmoj/cptbox/handlers.py	100.00% <0.00%> (+26.31%)	⬆️
dmoj/cptbox/isolate.py	89.63% <0.00%> (+38.41%)	⬆️
dmoj/control.py	100.00% <0.00%> (+64.70%)	⬆️

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