boot/initrafms.go: use systemctl reboot instead of /sbin/reboot

[valentindavid]

We are trying to get rid of /sbin in favor for /usr/sbin in core-initrd.

Also we should call directly systemctl which should always be in PATH.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 0.00%. Comparing base (5e81c9d) to head (cfef428). Report is 76 commits behind head on master.

Additional details and impacted files

@@       Coverage Diff       @@
##   master   #14778   +/-   ##
===============================
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[valentindavid]

Unfortunately the spread tests do not test this correctly. Since the only tests that trigger this do not repack pc-kernel with a fixed snap-bootstrap.

[github-actions[bot]]

Wed Apr 9 16:55:52 UTC 2025 The following results are from: https://github.com/canonical/snapd/actions/runs/14329796985

Failures:

Executing:

• openstack:opensuse-15.6-64:tests/main/security-device-cgroups-required-or-optional
• openstack:opensuse-15.6-64:tests/main/progress