Fix Snapcraft CI release, use new authentication method

.gitlab-ci.yml

@@ -188,7 +188,7 @@ release-snap:
     # Publish snap package
     - echo "Publishing snap package..."
     - echo "$SNAPCRAFT_LOGIN" | base64 -d > snapcraft.login
-    - snapcraft login --with snapcraft.login
+    - snapcraft whoami
     - snapcraft push --release=stable ffsend_*_amd64.snap
   artifacts:
     name: ffsend-snap-x86_64