executors: disallow access to /proc

[Xyene]

/proc/self is difficult/impossible to properly support under Landlock, but runtimes should not be relying on its presence anyway, as it is not POSIX.

[codecov-commenter]

Codecov Report

Merging #910 (1885e8c) into master (de73d84) will decrease coverage by 33.94%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #910       +/-   ##
===========================================
- Coverage   86.27%   52.33%   -33.95%     
===========================================
  Files         137      137               
  Lines        4640     4634        -6     
===========================================
- Hits         4003     2425     -1578     
- Misses        637     2209     +1572     

Impacted Files	Coverage Δ
dmoj/executors/mixins.py	92.47% <ø> (-2.21%)	:arrow_down:
dmoj/executors/C.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/D.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/GO.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/ADA.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/AWK.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/C11.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/F95.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/ICK.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/LUA.py	0.00% <0.00%> (-100.00%)	:arrow_down:
... and 81 more

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