ci: avoid uploading alpha appcasts

Currently alpha appcasts get uploaded. It turns out these overwrite the
standard appcast, so even though no one will update to the alpha
channel, everyone will miss regular updates while the latest build was
an alpha one.

Ideally we should combine the source of truth for both the alpha and
release channels, but as no one is using the alpha channel for yet let's
stop uploading it for now.

Test plan:

![eyes](https://media1.giphy.com/media/v1.Y2lkPTc5MGI3NjExeGNwdDk0dmdscjlkZnd6eGxhcjJzdDBsYndmc2t2cnlpZDNxZnZhYSZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/gKHGnB1ml0moQdjhEJ/giphy.gif)

.github/workflows/build-app.yml

@@ -296,5 +296,5 @@ jobs:
           aws s3 cp "$DMG_NAME" "s3://${SPARKLE_S3_BUCKET}/${PREFIX}${DMG_NAME}"
           if [[ "$IS_ALPHA" != "true" ]]; then
             aws s3 cp "$DMG_NAME" "s3://${SPARKLE_S3_BUCKET}/${PREFIX}EXO-latest.dmg"
+            aws s3 cp appcast.xml "s3://${SPARKLE_S3_BUCKET}/${PREFIX}appcast.xml" --content-type application/xml --cache-control no-cache
           fi
-          aws s3 cp appcast.xml "s3://${SPARKLE_S3_BUCKET}/${PREFIX}appcast.xml" --content-type application/xml --cache-control no-cache