Trigger CI only on PR branch commits

Why these changes are being introduced:

Formerly, CI was running time a commit was pushed to Github,
for any branch, regardless of an associated PR.

How this addresses that need:

An update to `.github/workflows/ci.yml` makes it so CI is
only run when a PR is opened and/or a commit is pushed to
a branch with an open PR.

Side effects of this change:
* CI will not run for pushes to a branch without an open
PR.

Relevant ticket(s):
* None

Author: ghukill

.github/workflows/ci.yml

@@ -1,5 +1,8 @@
 name: CI
-on: push
+on:
+  pull_request:
+    paths-ignore:
+      - '.github/**'
 jobs:
   test:
     # TODO: move this into a shared workflow, https://github.com/MITLibraries/.github/tree/main/.github/workflows