[DPE-4977] Run tests against juju 3.6 on a nightly schedule

[shayancanonical]

Issue

1. We are not running our integration tests against juju 3.6
2. We are not running our integration tests against the latest lxd version

Solution

1. Run integration tests on juju 3.6 on a nightly schedule only
2. Do not specify lxd-snap-channel to always run tests against the latest lxd version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.81%. Comparing base (f9032b8) to head (55fe81c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #601   +/-   ##
=======================================
  Coverage   70.81%   70.81%           
=======================================
  Files          12       12           
  Lines        3039     3039           
  Branches      537      537           
=======================================
  Hits         2152     2152           
  Misses        771      771           
  Partials      116      116           

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[taurus-forever]

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