Deduplicate build action runs

The previous action trigger of `on: [push, pull_request]` looked like it was
double dipping -- every time you pushed a new set of commits to a pull request,
the workflow triggered twice.

This instead runs the actions on new pull requests, or on synchronize/reopened
events. The synchronize event occurs when new commits are received.

Additionally, this still runs the test whenever someone pushes to master, which
shouldn't be done...

.github/workflows/build-containers.yml

@@ -1,6 +1,11 @@
 name: Build SPRAS Containers
 
-on: [push, pull_request]
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+  push:
+    branches:
+      - master
 
 jobs:
   build-and-remove-omics1: