Fix FD_Fail test in nix sandbox

[numinit]

Summary:

/etc/mtab doesn't exist in the Nix build sandbox since /etc doesn't
exist. However, /dev/zero is more common on UNIX systems and does.

Checklist:

• The PR addresses a single issue. If it can be divided into multiple independent PRs, please do so.
• The PR is divided into a logical sequence of commits OR a single commit is sufficient.
• There are no unnecessary commits (e.g. commits fixing issues in a previous commit in the same PR).
• Each commit has a meaningful message documenting what it does.
• All messages added to the codebase, all comments, as well as commit messages are spellchecked.
• The code is commented OR not applicable/necessary.
• The patch is formatted via clang-format OR not applicable (if explicitly overridden leave unchecked and explain).
• There are test cases for the code you added or modified OR no such test cases are required.

[ccadar]

@numinit sorry for the delay on merging this. The patch is fine, but could you please rebase it and push it again, as I'm not sure why the CI didn't get triggered properly. Thanks!

[codecov]

Codecov Report

Merging #1471 (53af6a5) into master (ec81fd1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1471   +/-   ##
=======================================
  Coverage   69.35%   69.35%           
=======================================
  Files         155      155           
  Lines       18481    18481           
  Branches     4100     4100           
=======================================
  Hits        12818    12818           
  Misses       3886     3886           
  Partials     1777     1777           

[ccadar]

Thanks, @numinit