ci: Add snapcraft publish action

[ottnorml]

For this action to work the secrets SNAPCRAFT_STORE_LOGIN must be available.

For the snapcraft publish action to work, the SNAPCRAFT_STORE_LOGIN secret must be available. It can be created as described in the documentation of the action (see link). It should be noted that the name of the secret is different from the one in the documentation to be more readable.

$ snapcraft export-login --snaps=PACKAGE_NAME \
      --acls package_access,package_push,package_update,package_release \
      exported.txt

refs https://github.com/zidoro/pomatez/pull/412, https://github.com/zidoro/pomatez/pull/418