CI: run regression-tests.yml on all PRs & primary branches (JuliaDocs#2694)

Author: fingolfin

.github/workflows/regression-tests.yml

@@ -1,6 +1,14 @@
 name: 'Regression Tests'
 on:
   workflow_dispatch:
+  push:
+    branches:
+      - master
+      - release-*
+      # You can suffix the branch name with '-runci' to run the CI on that branch
+      # without having to open a pull request. This is useful for long-lived branches.
+      - '*/*-runci'
+    tags: '*'
   pull_request:
     paths:
       - '.github/workflows/regression-tests.yml'