ci: only run soak tests on master branch

Spun off from getodk#1315.

Soak tests are slow, and are only likely to break occasionally.  Re-enabling them on branches when working on specific features, e.g. `slonik` or `pg` upgrades, would be sensible.

.github/workflows/soak-test.yml

@@ -1,6 +1,8 @@
 name: Soak Test
 
-on: push
+on:
+  push:
+    branches: master
 
 jobs:
   soak-test: