Allow use of bash as a script interpreter

[iansltx]

For #24470.

Haven't tested E2E yet as this requires building a new fleetd (which can/should be released before the server changes get tagged...if the server gets released first a bash script execution will error out but won't have any ill effects otherwise) + testing with local TUF (and also means this needs to go in after fleetd is unfrozen).

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.
• Added/updated automated tests
• A detailed QA plan exists on the associated ticket (if it isn't there, work with the product group's QA engineer to add it)
• Manual QA for all new/changed functionality
• For Orbit and Fleet Desktop changes:
  • Orbit runs on macOS, Linux and Windows. Check if the orbit feature/bugfix should only apply to one platform (runtime.GOOS).
  • Manual QA must be performed in the three main OSs, macOS, Windows and Linux.
  • Auto-update manual QA, from released version of component to new version (see tools/tuf/test).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.59%. Comparing base (6977ec9) to head (41c6668).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25449      +/-   ##
==========================================
- Coverage   63.59%   63.59%   -0.01%     
==========================================
  Files        1619     1619              
  Lines      154939   154938       -1     
  Branches     4038     3987      -51     
==========================================
- Hits        98530    98529       -1     
  Misses      48641    48641              
  Partials     7768     7768              

Flag	Coverage Δ
frontend	53.21% <ø> (ø)

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