Make CI easier to run in forks

This makesthe CI workflow easier to run in forks, for those who
wish to run it in feature branches that don't (or don't yet) have a
PR, by making it so that, in addition to `main`, branch names that
have `run-ci` as a non-trailing component also run it on `push`.

Author: EliahKagan

.github/workflows/ci.yml

@@ -2,9 +2,13 @@ name: CI
 
 on:
   push:
-    branches: [ main ]
+    branches:
+    - main
+    - "run-ci/**"
+    - "**/run-ci/**"
   pull_request:
-    branches: [ main ]
+    branches:
+    - main
 
 jobs:
   build-and-test-linux: