GitHub Actions: Don't build tags

Our workflow script does not currently work with tags, and there is no
point to building tags anyhow, since we do not use the CI system to spin
official builds.

.github/workflows/build.yml

@@ -1,6 +1,13 @@
 name: Build
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  push:
+    branches:
+      - '**'
+    tags-ignore:
+      - '**'
+  pull_request:
+  workflow_dispatch:
 
 jobs:
   linux: