Build on push to branches, not tags.

According to the documentation, by not specifying anything in the `push`
event, GitHub Actions will run the workflow for both branches and tags,
but by specifying `branches` alone, it will _only_ run the workflow for
branches and _not_ tags [1].

We want to build on pushes to _all_ branches so we can give folks a jar
to try out for feature requests and bug fixes. We don't want to build on
tags, however, because they don't provide any value in the current build
pipeline. If the version was derived from Git tags, it could make sense
to build on tags, but that's not where we are.

As for the `**` glob pattern instead of just `*`, the documentation says
that the latter does not match forward slashes [2], and while our naming
convention for branches doesn't use slashes right now, there's no reason
that they shouldn't be viable later down the road.

---

[1] https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushbranchestagsbranches-ignoretags-ignore
[2] https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#filter-pattern-cheat-sheet

.github/workflows/build.yml

@@ -3,6 +3,8 @@ name: build
 on:
   workflow_dispatch:
   push:
+    branches:
+      - '**'
 
 jobs:
   build: