release: add optional fallback to /etc/initrd-release

[bboozzoo]

Add /etc/initrd-release documented in os-release(5) as an optional fallback, for cases when the initrd lacks required symlinks.

Cherry picked from #15030

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Report is 185 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #15036   +/-   ##
=========================================
  Coverage          ?   78.23%           
=========================================
  Files             ?     1161           
  Lines             ?   154326           
  Branches          ?        0           
=========================================
  Hits              ?   120738           
  Misses            ?    26154           
  Partials          ?     7434           

Flag	Coverage Δ
unittests	78.23% <100.00%> (?)

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[github-actions]

Thu Feb 6 17:20:45 UTC 2025
The following results are from: https://github.com/canonical/snapd/actions/runs/13177507791

Failures:

Preparing:

• openstack:opensuse-tumbleweed-64
• openstack:opensuse-tumbleweed-64
• openstack:opensuse-tumbleweed-64
• google-arm:ubuntu-core-22-arm-64

Executing:

• openstack:centos-9-64:tests/main/degraded
• openstack:debian-12-64:tests/main/microk8s-smoke:edge
• openstack:debian-sid-64:tests/main/microk8s-smoke:edge
• openstack:fedora-40-64:tests/main/degraded
• google:ubuntu-25.04-64:tests/main/security-device-cgroups:kmsg
• google:ubuntu-25.04-64:tests/main/cgroup-devices-v2
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-strict-enforced
• google:ubuntu-25.04-64:tests/main/security-device-cgroups:uinput
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-serial-port
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-self-manage
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-helper
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-required-or-optional

Restoring:

• openstack:fedora-40-64:tests/main/revert:fake
• openstack:fedora-40-64:tests/main/
• openstack:fedora-40-64
• openstack:opensuse-tumbleweed-64
• openstack:opensuse-tumbleweed-64
• google-arm:ubuntu-core-22-arm-64
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-strict-enforced

[miguelpires]

lgtm

[Meulengracht]

Thank you