snap: use exported credentials from 'SNAPCRAFT_STORE_CREDENTIALS'

The old 'login --with' method has been deprecated and is no longer
supported.  Switch to the new method of using a previously-exported
credential, by reading it from the 'SNAPCRAFT_STORE_CREDENTIALS'
environment variable.

https://snapcraft.io/docs/snapcraft-authentication#heading--using

Change-Id: Icd3f0716f4f82df62891cc71dc7e94c92eeff558
2 files changed