Replace fgrep with grep -F to avoid a warning

[mattmccutchen]

Today, I updated the dom0 kernel of my Qubes 4.3 system and saw a bunch of "warning: fgrep is obsolescent; using grep -F" messages. This change (if backported) should stop those messages, FWIW. Untested but hopefully obviously correct.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.63%. Comparing base (d7df71c) to head (89e39bd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #207   +/-   ##
=======================================
  Coverage   72.63%   72.63%           
=======================================
  Files          10       10           
  Lines        1162     1162           
=======================================
  Hits          844      844           
  Misses        318      318           

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