CI: combine dual workflows into one

Author: ctrueden

.github/workflows/build-pr.yml

@@ -1,6 +1,9 @@
 name: build
 
 on:
+  pull_request:
+    branches:
+      - master
   push:
     branches:
       - master

.github/workflows/build-main.yml .github/workflows/build.yml

@@ -1,5 +1,5 @@
 [![](https://img.shields.io/maven-central/v/org.scijava/scijava-common.svg)](https://search.maven.org/#search%7Cgav%7C1%7Cg%3A%22org.scijava%22%20AND%20a%3A%22scijava-common%22)
-[![](https://github.com/scijava/scijava-common/actions/workflows/build-main.yml/badge.svg)](https://github.com/scijava/scijava-common/actions/workflows/build-main.yml)
+[![](https://github.com/scijava/scijava-common/actions/workflows/build.yml/badge.svg)](https://github.com/scijava/scijava-common/actions/workflows/build.yml)
 [![developer chat](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://imagesc.zulipchat.com/#narrow/stream/327237-SciJava)
 
 SciJava Common is a common library for SciJava software. It provides a