fix(2.7): allow skopeo to use an insecure policy

[lengau]

Fixes this error:

 + charmcraft upload-resource gh-ci-charmcraft-charm example-image --image=docker://hello-world@sha256:18a657d0cc1c7d0678a3fbea8b7eb4918bba25968d3e1b0adebfa71caddbc346
Uploading resource from image charm/co44x4qvu12a2ff5a1v04haywbiyhc0uv6erc/example-image @ docker://hello-world@sha256:18a657d0cc1c7d0678a3fbea8b7eb4918bba25968d3e1b0adebfa71caddbc346.
Checking if manifest is already uploaded
Remote image not found, getting its info from local registry.
Image not found locally. Passing path directly to skopeo.
Running Skopeo
:: time="2024-08-26T18:39:59Z" level=fatal msg="Error loading trust policy: open /etc/containers/policy.json: no such file or directory"
Error while running /snap/charmcraft/x1/libexec/charmcraft/skopeo (return code 1)
Full execution log: '/root/.local/state/charmcraft/log/charmcraft-20240826-183956.163959.log'
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