Disable agent auto update in snaps

[matheushent]

What

This PR removes the configuration task-self-update-interval-seconds from the jobbergate agent snap.

Why

The agent is not supposed to update itself when deployed by the snap.

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Peer Review

Please follow the upstream omnivector documentation concerning
peer-review guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.58%. Comparing base (119bc01) to head (3a84a63).
Report is 1 commits behind head on release/5.4.

Additional details and impacted files

@@             Coverage Diff              @@
##           release/5.4     #670   +/-   ##
============================================
  Coverage        92.58%   92.58%           
============================================
  Files               85       85           
  Lines             4490     4490           
============================================
  Hits              4157     4157           
  Misses             333      333           

Flag	Coverage Δ
agent	92.80% <ø> (ø)
api	95.33% <ø> (ø)
cli	88.62% <ø> (ø)
core	96.31% <ø> (ø)

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