desktop: cherry-pick pc-kernel from different channel

To get 25.10 Desktop ISOs with TPMFDE bits, we need matching pc-kernel
and snapd otherwise we get errors like so when running
`snap prepare-image`:

WARNING: the kernel for the specified UC20+ model does not carry
assertion max formats information, assuming possibly incorrectly the
kernel revision can use the same formats as snapd
error: snapd 2.68+ is not compatible with a kernel containing snapd
prior to 2.68

Use the "dangerous" model, which allows overriding the channel, and pick
up the matching pc-kernel which is not yet on 25.10/stable, where the
non-dangerous model would expect to find it.

live-build/ubuntu/hooks/020-ubuntu-enhanced-sb.binary

@@ -38,6 +38,9 @@ fi
 # env SNAPPY_STORE_NO_CDN=1 snap known --remote model series=16 brand-id=canonical model=ubuntu-classic-2410-amd64 > config/classic-model.model
 model=/usr/share/livecd-rootfs/live-build/${PROJECT}/ubuntu-classic-amd64.model
 
+# see below note about "dangerous" model
+CHANNEL=${CHANNEL:-stable}
+
 channel=""
 if [ -n "${CHANNEL:-}" ]; then
     channel="--channel $CHANNEL"
@@ -47,8 +50,17 @@ reset_snapd_state chroot
 
 # Set UBUNTU_STORE_COHORT_KEY="+" to force prepare-image to fetch the latest
 # snap versions regardless of phasing status
+
+# this is the normal prepare-image invocation.  This is not used right now as
+# the model in question is the "dangerous" model so that we can override the
+# channel of pc-kernel to get a matching set of snaps.
+#   env SNAPPY_STORE_NO_CDN=1 UBUNTU_STORE_COHORT_KEY="+" snap prepare-image \
+#       --classic $model $channel chroot
+# so instead we're doing this, including forcing channel to stable for
+# everything but pc-kernel.
 env SNAPPY_STORE_NO_CDN=1 UBUNTU_STORE_COHORT_KEY="+" snap prepare-image \
-    --classic $model $channel chroot
+    --classic $model $channel --snap=pc-kernel=25.04/edge chroot
+
 mv chroot/system-seed/systems/* chroot/system-seed/systems/enhanced-secureboot-desktop
 rm -rf chroot/var/lib/snapd/seed
 mv chroot/system-seed chroot/var/lib/snapd/seed

live-build/ubuntu/ubuntu-classic-amd64.model

@@ -2,93 +2,88 @@ type: model
 authority-id: canonical
 series: 16
 brand-id: canonical
-model: ubuntu-classic-2510-amd64
+model: ubuntu-classic-2510-amd64-dangerous
 architecture: amd64
 base: core24
 classic: true
 distribution: ubuntu
-grade: signed
+grade: dangerous
 snaps:
   -
-    default-channel: classic-25.10/stable
+    default-channel: classic-25.10/edge
     id: UqFziVZDHLSyO3TqSWgNBoAdHbLI4dAH
     name: pc
     type: gadget
   -
-    default-channel: 25.10/stable
+    default-channel: 25.10/edge
     id: pYVQrBcKmBa0mZ4CCN7ExT6jH8rY1hza
     name: pc-kernel
     type: kernel
   -
-    default-channel: latest/stable
+    default-channel: latest/edge
     id: amcUKQILKXHHTlmSa7NMdnXSx02dNeeT
     name: core22
     type: base
   -
-    default-channel: latest/stable
+    default-channel: latest/edge
-    id: dwTAh7MZZ01zyriOZErqd1JynQLiOGvM
-    name: core24
-    type: base
-  -
-    default-channel: latest/stable
     id: PMrrV4ml8uWuEUDBT8dSGnKUYbevVhc4
     name: snapd
     type: snapd
   -
-    default-channel: latest/stable
+    default-channel: latest/edge
     id: EISPgh06mRh1vordZY9OZ34QHdd7OrdR
     name: bare
     type: base
   -
-    default-channel: 1/stable/ubuntu-25.04
+    default-channel: latest/edge
     id: EI0D1KHjP8XiwMZKqSjuh6W8zvcowUVP
     name: firmware-updater
     type: app
   -
-    default-channel: 1/stable/ubuntu-25.04
+    default-channel: latest/edge
     id: FppXWunWzuRT2NUT9CwoBPNJNZBYOCk0
     name: desktop-security-center
     type: app
   -
-    default-channel: 1/stable/ubuntu-25.04
+    default-channel: latest/edge
     id: aoc5lfC8aUd2VL8VpvynUJJhGXp5K6Dj
     name: prompting-client
     type: app
   -
-    default-channel: 2/stable/ubuntu-25.04
+    default-channel: latest/edge
     id: gjf3IPXoRiipCu9K0kVu52f0H56fIksg
     name: snap-store
     type: app
   -
-    default-channel: latest/stable/ubuntu-25.10
+    default-channel: latest/edge
     id: jZLfBRzf1cYlYysIjD2bwSzNtngY0qit
     name: gtk-common-themes
     type: app
   -
-    default-channel: latest/stable/ubuntu-25.10
+    default-channel: latest/edge
     id: 3wdHCAVyZEmYsCMFDE9qt92UV8rC8Wdk
     name: firefox
     type: app
   -
-    default-channel: latest/stable/ubuntu-25.10
+    default-channel: latest/edge
     id: lATO8HzwVvrAPrlZRAWpfyrJKlAJrZS3
     name: gnome-42-2204
     type: app
   -
-    default-channel: latest/stable/ubuntu-25.10
+    default-channel: latest/edge
     id: IrwRHakqtzhFRHJOOPxKVPU0Kk7Erhcu
     name: snapd-desktop-integration
     type: app
 timestamp: 2025-05-01T12:00:00.0Z
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