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

All modified and coverable lines are covered by tests ✅

Project coverage is 52.33%. Comparing base (de73d84) to head (1885e8c).
Report is 378 commits behind head on master.

Additional details and impacted files

@@             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     

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