github: Don't run builds for tags, only pushes to branches

There's no need to rerun all the builds when pushing a tag.

Author: mstorsjo

.github/workflows/build.yml

@@ -1,6 +1,8 @@
 name: Builds
 on:
   push:
+    branches:
+      - '**'
   schedule:
     - cron: '0 0 * * *'