CI: Build binaries for tag pushes

GitHub Actions was set to only build for branch pushes (only master
branch due to the branch filter), PRs, and manual workflow dispatches.

Now, GitHub Actions will also build for tag pushes.

This helps to ensure Regular releases
get signed binaries built for them.

(GitHub Actions is set to only *sign* the binaries for push events.
Tag creations/pushes will generate push events, so tag pushes should
indeed make signed binaries, not unsigned ones.)

.github/workflows/build.yml

@@ -4,6 +4,8 @@ on:
   push:
     branches:
       - 'master'
+    tags:
+      - '**'
   pull_request:
   workflow_dispatch: