Make the main CI workflow easier to run in forks

This adds patterns besides the literal name `main` so CI runs on
`push` to any branch that has `run-ci` as a non-trailing component.
It also adds a `workflow_dispatch` trigger so the workflow can be
run (on any branch) manually from the Actions tab.

These are to make the workflow easier to run in forks, roughly
analogous to GitoxideLabs/gitoxide#1507.

Author: EliahKagan

.github/workflows/ci.yml

@@ -8,6 +8,9 @@ on:
   push:
     branches:
     - main
+    - 'run-ci/**'
+    - '**/run-ci/**'
+  workflow_dispatch:
 
 env:
   RUST_BACKTRACE: 1