Systemd based runner default on core (bugfix)

[Hook25]

Description

The current subshell runner will be broken on core (in some scenarios) starting 6Jan with the release of snapd beta to stable. This comes in the form of a cgroup tracking error when calling a snap from a snap like the following:

/system.slice/snap.checkbox-riverside.remote-slave.service is not a snap cgroup for tag  snap.bluez.hcitool 

For this reason we must make the new systemd-based runner default on core. Note that the option to use the old runner will remain via the feature flag as the value from the user will always be respected. This only changes the default value from False (never enabled by default) to None (sometimes enabled by default).

This PR also updates the build repo for plz-run to my Github fork (pending move under the Canonical org) as we need to be able to update this tool while the maintainer is OOO. Note that this is necessary as I have landed a bugfix there.

Finally this pr also adds a small bugfix (there is a typo in the PAM module to use, to double check simply ls /etc/pam.d, there is no systemd-login, its system-login!)

Resolved issues

Fixes: https://warthogs.atlassian.net/browse/CHECKBOX-2142

Documentation

N/A

Tests

Tested this locally to verify the bugfix fixes the regressions reported by QA.
Snap builds:

• https://github.com/canonical/checkbox/actions/runs/20227521842
• https://github.com/canonical/checkbox/actions/runs/20227530435

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 53.71%. Comparing base (1b7ffef) to head (da31062).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
checkbox-ng/plainbox/impl/session/state.py	85.71%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (87.50%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2247      +/-   ##
==========================================
+ Coverage   53.69%   53.71%   +0.01%     
==========================================
  Files         401      401              
  Lines       43145    43153       +8     
  Branches     7996     7999       +3     
==========================================
+ Hits        23167    23179      +12     
  Misses      19161    19161              
+ Partials      817      813       -4     

Flag	Coverage Δ
checkbox-ng	71.57% <87.50%> (+0.03%)	⬆️
checkbox-support	65.29% <ø> (ø)
provider-base	30.90% <ø> (ø)
provider-certification-client	57.14% <ø> (ø)
provider-certification-server	57.14% <ø> (ø)
provider-genio	96.90% <ø> (ø)
provider-gpgpu	93.14% <ø> (ø)
provider-iiotg	100.00% <ø> (ø)
provider-resource	39.30% <ø> (ø)
provider-sru	97.97% <ø> (ø)

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[fernando79513]

LGTM +1!