Modifying a build-snap doesn't trigger a change

[dilyn-corner]

Bug Description

If the channel of a specific build snap is modified, a rebuild of the specific part will not be triggered.

To Reproduce

Create a snap with a part declaring a build-snap as so:

    build-snaps: [foo/bar/baz]

Build the snap and then modify bar/baz to some other valid track/risk for the foo snap. Rerun the build and notice that the build step for the part is not retriggered.

Environment

Ubuntu 25.04, building with snapcraft 8.11.1 and LXD latest/edge.

snapcraft.yaml

name: test # you probably want to 'snapcraft register <name>'
base: core24 # the base snap is the execution environment for this snap
version: '0.1' # just for humans, typically '1.2+git' or '1.3.2'
summary: Single-line elevator pitch for your amazing snap # 79 char long summary
description: |
  This is my-snap's description. You have a paragraph or two to tell the
  most important story about your snap. Keep it under 100 words though,
  we live in tweetspace and your description wants to look good in the snap
  store.

grade: devel # must be 'stable' to release into candidate/stable channels
confinement: devmode # use 'strict' once you have the right plugs and slots

parts:
  my-part:
    # See 'snapcraft plugins'
    plugin: nil
    build-environment:
      - FOO: bar
    # Get past the point where build-snaps are installed in the env and change 12 to e.g. 22, do another build
    build-snaps: [node/12/stable]

Relevant log output

 snapcraft pull --shell
Starting snapcraft, version 8.11.1.post53
Logging execution to '/home/dilyn/.local/state/snapcraft/log/snapcraft-20250828-141221.610376.log'
Launching managed ubuntu 24.04 instance...
Starting instance 
Starting snapcraft, version 8.11.1.post53
Logging execution to '/tmp/snapcraft.log'
Initialising lifecycle
Installing build-packages
Installing build-snapssn
Launching shell on build environment...
snapcraft-test-amd64-4235994 ../project# snap list
Name       Version             Rev    Tracking       Publisher   Notes
core       16-2.61.4-20250626  17247  latest/stable  canonical✓  core
core24     20250618            1055   -              canonical✓  base
node       12.22.12            6195   12/stable      iojs✓       classic
snapcraft  8.11.1.post53       x1     -              -           classic
snapd      2.71                25202  latest/stable  canonical✓  snapd
snapcraft-test-amd64-4235994 ../project# exit

Additional context

Noticed this when working around an issue where I needed to update the version of node in my build environment. I was using node from 12/stable but needed at least node from 18/stable. s/12/18/ was insufficienet to resolve the issue, because the modified snap channel was not considered when validating the build environment.