Charmcraft snap thinks /cache is the cache directory if snapd doesn't set SNAP_USER_COMMON

[lengau]

Bug Description

If snapd fails to set $SNAP_USER_COMMON](https://bugs.launchpad.net/snapd/+bug/2090938) in the environment, charmcraft gets /cache as its [$XDG_CACHE_HOME`, which is generally not writable.

Related:

• https://bugs.launchpad.net/snapd/+bug/2090938
• Craft tools get executed with an empty $PATH operator-workflows#496

To Reproduce

With snapd 2.66.1:

echo '$SNAP/bin/python -c "from charmcraft import env; print(env.get_host_shared_cache_path())"' | PATH=/snap/bin /usr/bin/snap run --shell charmcraft

Environment

$ snap version
snap    2.66.1+25.04
snapd   2.66.1+25.04
series  16
ubuntu  25.04
kernel  6.11.0-9-generic

charmcraft.yaml

n/a

Relevant log output

See: https://github.com/canonical/k8s-operator/actions/runs/12143320826/job/33859924966